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In the real world, humans have tamed horses and use them as mounts. Horses have very specific characteristics that make them a viable mount, such as size, back-strength, stamina, diet, behavior, etc. That is also the reason why we don't ride animals such as deer and zebras.

In my fantasy world, humans have domesticated Species X to be used as flying mounts. Species X are feathered animals that have six limbs, four for land locomotion plus two wings.

What anatomical characteristics would these creatures need in order to be practical flying mounts?

  • Mount: an animal that people ride in order to travel arbitrary distances
  • Flying: capable of self-powered and controlled movement through the air

EDIT: If such a creature seems unfeasible, an answer stating so with proper arguments is as valid as any other.

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    $\begingroup$ I'd just go for a Pegasus. It's a horse with wings, has four legs so the six limbs are there. Depending on your world you could have eagles, drakes, insects or even griffons. Or a horse and a catapult :). $\endgroup$ – Snowlockk Jan 25 '17 at 14:26
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    $\begingroup$ They probably need to be huge, since flying is not small feat, anatomically speaking. Birds have hollow bones and very large chest muscles, everything is tweaked to be light. Just look at the larger birds that require several meters of wingspan to fly. Now think of a bird that needs to lift ~80Kg, while still being able to retain maneuverability. $\endgroup$ – r41n Jan 25 '17 at 14:31
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    $\begingroup$ @Snowlockk I really doubt a traditional pegasus would actually be capable of flight, much less carrying a person while doing so. $\endgroup$ – Miguel Bartelsman Jan 25 '17 at 14:31
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    $\begingroup$ If your world has hovering islands, How do they work? Can the same principle apply to those animals? $\endgroup$ – Westside Tony Jan 25 '17 at 16:28
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    $\begingroup$ Much overlap with worldbuilding.stackexchange.com/questions/819/… $\endgroup$ – Agent_L Jan 26 '17 at 11:38

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I don't think this is possible. I'm assuming you want a realistic answer. A flying mount looks unfeasible on Earth, or on an Earth-like setting, because it would have to be both extremely light and extremely strong for its size (indeed, like real life flying animals are). This is even before you consider riding the animal.

In general, consider that weight goes up with volume (cube of linear size) while muscle strength and wings go up with area (square of linear size) so the bigger it is, the weaker it'll proportionally become (square-cube law).

Earth's largest-ever flying animal was Quetzalcoatlus, which is estimated to have had a 11 meter wingspan and a weight of about 200 kg. We don't know exactly how it flew, and some have even proposed that it didn't fly at all (maybe it could glide?). The largest-ever flying bird was Argentavis (6 m wingspan, 70 kg). According to its article in Wikipedia, "a wing loading of 25 kg/m² is considered the limit for avian flight".

A small-breed modern horse weighs about 400 kg, so for a horse to fly with 100 kg of cargo (rider plus whatever) it would have to have 20 m² of wing surface (each wing being 5 by 2 meters!) and it would also probably have to have hollow bones, massive wing muscles and an extremely fast, bird-like metabolism. The bones on the other hand would have to be able to support the 500 kg of the loaded animal when on the ground. The wing muscles would add mass, so the rest would have to be proportionally small and weak. And the fast metabolism would mean the animal must be fed often and abundantly (and therefore could not fly long distances).

So I believe a Pegasus-like mount is impossible on Earth. It could be possible, maybe, if you're willing to make some concessions, such as having only very light riders (a caste of messenger midgets?), accepting that the animal will be fragile and die easily so it can only be used for very very important purposes, and/or somehow managing to enhance the animal using nanotech, for example, to make extremely thin bones extra-strong or to increase metabolic efficiency (this could be hand-waved by implying an advanced civilization tinkered with them ages ago and then left).

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    $\begingroup$ While I agree to your general analysis, I've got a proposal: Change the atmosphere! As hovering islands are already a stretch / magic, it might be possible to handwave away the other effects (gas toxicity etc.) of increased atmospheric pressure, but flying/gliding gets increasingly easier in a denser atmosphere. Let your creature then feed of something it has to transport (think of any bird of prey, but can also be herbivorous; needs to transport food to remote nesting site to protect offspring) to explain the ability for supporting extra weight in flight and it should work. $\endgroup$ – Benedikt Jan 25 '17 at 15:54
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    $\begingroup$ Why isn't this answer "Yes, use the Quetzalcoatlus"? According to wikipedia it is estimated to have weighed 1200 lbs and was capable of flight "up to 80 miles an hour for 7 to 10 days at altitudes of 15,000 feet". Seems like adding another 100 lbs wouldn't make it flightless. Make the humans into "elves" instead and you can cut their weight in half. Add 10-15% to atmospheric pressure and subtract 10-15% to gravity and you'd be more than fine. To scale, it'd be like an eagle flying off with a cat. No? $\endgroup$ – Shane Jan 25 '17 at 20:10
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    $\begingroup$ "In 2010, Mike Habib, [...], and Mark Witton, [...] undertook further investigation into the claims of flightlessness in large pterosaurs. After factoring wingspan, body weight, and aerodynamics, computer modelling led the two researchers to conclude that Q. northropi was capable of flight "up to 80 miles an hour for 7 to 10 days at altitudes of 15,000 feet". Habib further suggested a maximum flight range of 8,000 to 12,000 miles for Q. northropi. $\endgroup$ – Shane Jan 25 '17 at 20:54
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    $\begingroup$ Henderson's work was also further criticized by Witton and Habib in another study, who pointed out that although Henderson used excellent mass estimations, they were based on outdated pterosaur models, and that anatomical study of Q. northropi and other large pterosaur forelimbs show a higher degree of robustness than would be expected if they were purely quadrupedal." $\endgroup$ – Shane Jan 25 '17 at 20:54
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    $\begingroup$ Yes, there is the Quetzalcoatlus northropi as well as the Quetzalcoatlus sp. If you'll notice in those quotes, they only mentioned the the Quetzalcoatlus northropi. It is 540 KG and flies up to 80 miles an hour for 7 to 10 days at altitudes of 15,000 feet. The smaller one is irrelevant. Also, and this is very important: The op did NOT ask for a creature resembling a horse. They asked: What anatomical characteristics would these creatures need in order to be practical flying mounts? The answer is that it could have very similar anatomical characteristics to the Quetzalcoatlus. $\endgroup$ – Shane Jan 25 '17 at 23:51
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I don't think a classic Pegasus/dragon animal will work (reasons see other answers).

But how about a "lighter-than-air" approach?

Your (gigantic) animal will split water into oxygen (can maybe be stored in special organs for fight-or-flight situations) and hydrogen. The hydrogen is then stored in large bladders, allowing the animal to float. It then can use huge firefly-like wings to "paddle" around (jellyfish with fireflywings).

A more elegant and maybe faster way would be a body aerodynamically shaped (think about a fat manta/stingray). It then would use muscles to squeeze the bladders, reducing its volume (and therefore its uplift), starting to fall. By adjusting its body-position it can than turn the falling motion into a gliding motion. Once it gets to low it releases the muscle-tension and starts to float back up. Repeat until target is reached.

TLDR: A manta-airship instead of horse-bird.

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    $\begingroup$ Or a gas-filled jelly fish that required the rider to wear a special suit. $\endgroup$ – Tim Jan 25 '17 at 15:57
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    $\begingroup$ As an alternative to water splitting, you could also consider an animal with a large population of methanogens in its digestive system that provide it with methane as a lifting gas. $\endgroup$ – Aliden Jan 25 '17 at 17:10
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    $\begingroup$ A fart-filled balloon. $\endgroup$ – Miguel Bartelsman Jan 25 '17 at 19:24
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    $\begingroup$ The Wind Whales of Ishmael, anyone? $\endgroup$ – pablodf76 Jan 25 '17 at 20:51
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    $\begingroup$ I like this idea, only of course you'll have to add some catastrophic gas-bladder sealing failures to the mythology of your species. $\endgroup$ – pablodf76 Jan 25 '17 at 20:53
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To point out an obvious evolutionary problem. The creature in question would have had to evolve to be able fly while weighing up to 150kg more than it usually does.

Hence why we only ride dragons, because that 150kg is a negligible proportion of their weight. Pegasus* and Gryphons just can't get off the ground carrying that much extra weight.

*Pegai perhaps, but that'd be Latin and he's Greek, so maybe Pegasodes. Pegasus was his name though, not his species.

Thanks to AlexP:

The plural of Pēgasos (Πήγασος) is Pēgasoi (Πήγασοι), which would be Latinized as Pegasoe.

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    $\begingroup$ The plural of Pēgasos (Πήγασος) is Pēgasoi (Πήγασοι), which would be Latinized as Pegasoe. $\endgroup$ – AlexP Jan 25 '17 at 15:32
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    $\begingroup$ Maybe the creature in question eats things (plants, animals) weighing over 80kg? which would explain the need to carry such weights. Or maybe it carries its younglings with it for a long time, marsupial-style. Perhaps it hunts at extremely large heights and the large wing area is to allow it to hunt in such low air densities. Nothing besides its number of limbs, feathers and lack of magic is set in stone. $\endgroup$ – Miguel Bartelsman Jan 25 '17 at 16:35
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    $\begingroup$ @MiguelBartelsman, only a dragon would eat that much and no flying creature carries its young, keeping the weight down is too important. Early invention of the hangglider after seeing birds in thermals is your best bet. $\endgroup$ – Separatrix Jan 25 '17 at 16:59
  • $\begingroup$ @Separatrix I still have hopes for a bird-drawn glider (who am I kidding) $\endgroup$ – Miguel Bartelsman Jan 25 '17 at 19:49
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    $\begingroup$ No need for natural evolution for that - evolved ability to fly, then selective breeding to be able to carry more. $\endgroup$ – Baldrickk Jan 26 '17 at 15:50
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I am looking at some flying animals right now, so the prospect of flying animals is not much of a stretch. Floating islands, though, are scarce. So: use whatever makes the hovering islands possible in your world to make your mounts fly.

You could really run with this and make it different. Pegasus / dragon riders / flying giant cats have been done done done but it would be fun to have the mounts be smaller than humans - ?maybe regular cats? - and really buoyant because they eat so much cavorite. You could ride one like a grumpy skateboard. Possibly your flight suit has a number of rings, each of which you clip to a different cat. Problems integral to dealing with cats would of course come into play, as well as new problems like the need for ceilings and a requirement that all flying cats are inside cats.

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    $\begingroup$ no need for cavorite. It suffices to tie a butter toast to their bellies. $\endgroup$ – John Dvorak Jan 26 '17 at 12:29
  • $\begingroup$ @PaulTIKI Unfortunately, it has been done before - see the villain CATapault in the league of super(redundant) heroes webcomic. He has telekinetic power over cats, which allows them to 'fly'. In one comic, he and one of the heros are hanging onto the cats to make an escape. $\endgroup$ – Baldrickk Jan 26 '17 at 15:49
  • $\begingroup$ OK, but still +1 for bringing up something unusual :) $\endgroup$ – Paul TIKI Jan 26 '17 at 17:11
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"The Flight of Dragons," by Peter Dickinson proposed a plausible evolutionary explanation of dragons. Plausible to my 13yo mind, at least. In it, he explained that dragon flight is a hybrid of wing based propulsion and lighter than air flight. The buoyancy was provided for by hydrogen, which also accounted for fire breath (ignited by an electric spark). The hydrogen was created through a chemical process involving acid, which accounted for dragon hoards as everything but gold would be corroded by dragon spit. I remember it being a pretty cohesive account.

Evolutionarily, I don't think this would be much more of a stretch than what we see on earth today. We have flying animals. We have animals that puff themselves up to ward off threats. We have serpents with amazingly elastic bodies. We have animals that produce acid. We have electric eels. I think if you work out something physically plausible the evolutionary explanation will be as good as any we have for existing animals.

Edit: And as for the mount aspect. It's been mentioned that birds will carry off small mammals, so there is always that. There could be explanations on the sexual selection side. Perhaps (this might have been in Flight of Dragons) flight and fire displays could have been involved in courtship. Females prefer males which could stay afloat longer with more of their hydrogen being burned, implying quick generation of gas or super buoyancy. Courtship could involve the males trying to lift the females and the females trying to escape downward.

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  • $\begingroup$ I remember "The Flight of Dragons" cartoon, it was great. There was a more recent movie called "Dragon's World" that to the same arguments. It was filmed documentary style. Very entertaining. Dragon flight as a combo of lighter than air and winged. Each unusual part was explained by showing the adaptive biology of various real species on earth. They covered everything but how they got 6 limbs. $\endgroup$ – Paul TIKI Jan 26 '17 at 17:14
  • $\begingroup$ I never saw those, but good comment. As for 6 limbs... how about wings being adapted from ribs? Alternatively, other 6 limbed animals. A whole phylum/class/order of which these flying mounts are a family/genus/species etc. $\endgroup$ – trevor cook Jan 27 '17 at 15:22
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Change the air density, then you can have Pegasi.
If you can increase the ambient air density significantly, without increasing local gravity, then a Pegasus-like solution would become much more feasible. (And since you already have floating islands, we know gravity can be weak, at least for some things.)

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Turn your mind back to Disney's Fantasia 2000, The second segment. A wonderful piece of music called "The Pines of Rome" with an animation of humpback whales that jump out of the water and fly up into the clouds. this would provide you with large animals that could carry people and fly from floating island to floating island.

Okay, this is far-fetched so lets look at how they got up there. Assume that the planet surface below is mostly covered in water. The islands hover at varying altitudes above the surface. What makes the Islands hover? The oceans contain large quantities of handwavium, which is also the principle component of the floating islands. The islands rise and fall slowly and majestically, and when they touch down in the ocean lots of little krill gorge themselves on the handwavium rich strata on the underside of the islands. The whales eat the krill, and for a while they gain the power of flight. Swarms of insects in the air also serve as handwavium rich snacks for the whales, prolonging their ability to fly. The whales return to the oceans for mating and caring for their young.

Gaining flight via diet is kind of like how Flamingos are pink because of the shrimp they eat.

The whales look a lot like humpbacks but have additional limbs as wings for control in flight, but that fold back flat against the body for movement in water. Propulsion is by means of a series of bladders that act as both lungs and like an octopus jet. Inhale through the nose, exhale out the other end. Not real fast, but majestic.

I know I left out the ability for land locomotion, but only because I like the idea of flying whales. If land locomotion is absolutely required, I guess you could use giant Sea Otters with feathers and wings. Use the same food cycle mechanism, but since otters are carnivorous, they could eat more normal birds in addition to the fish that eat the krill that contain the handwavium.

As mentioned by Will, Pegasus, gryphon, dragon, etc all have been done to death I like the idea of doing things a little differently

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Scott Westerfeld's Leviathan series had bioengineered zeppelins that were basically modified whales full of hydrogen.

Your average zeppelin had a lifting capacity of hundreds of thousands of pounds, so it seems like you could proportionally scale that down to an animal that is mostly hydrogen to carry a person around. The resulting beast would still end up a bit too big to pet though.

Scott's were very cool because they were built as a biome of various species and the leviathan produced its own hydrogen through digestive processes.

Anyway, that's not the feathered, limbed animal you asked for, but it might be a useful concept to incorporate. I always imagined Appa from Avatar as being mostly hydrogen underneath.. ; )

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  • $\begingroup$ I must find this Leviathan series now. Sounds interesting. $\endgroup$ – Paul TIKI Jan 26 '17 at 13:55
  • $\begingroup$ @PaulTIKI It's quite good. It's meant as YA fiction.. but he built some pretty solid alternative history. I just finished reading it to my son. $\endgroup$ – aaronP Jan 26 '17 at 18:20
  • $\begingroup$ Another thing I've fantasized about is a truly hollow structure.. (eg nothing but vacuum.) Even hydrogen, which is lighter than air still weighs something. If you could build a light structure that was literally filled with nothing (that could survive our crushing air pressure) you'd have the most efficient lighter than air craft possible. For that I'd hand wave carbon nanotubes at it. ; ) $\endgroup$ – aaronP Jan 26 '17 at 18:23
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Your planet needs to be either small or not very dense, with a fairly thick atmosphere. Per relevant XKCD a human with manageable-sized wings could fly on Titan, which is about 2.3% Earth's mass and 7% the volume, so about 30% the density. Scale a bit to some bird about the size of Quetzalcoatlus and it ought to be feasible.

You could have your planet be a hollow shell to adjust the overall gravitational pull for a given surface area, should you so wish, though you'd need a good explanation for why it doesn't collapse in on itself.

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  • $\begingroup$ Hollow shell planets are lots of fun - given an even distribution of mass around the planet, if you were to go within, you end up completely weightless, while experiencing normal gravity on the outside. $\endgroup$ – Baldrickk Jan 26 '17 at 15:52
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BIOLOGICAL JET PROPULTION

It was explored in "Alien Planet". It would be very difficult to evolve but once a species obtains it it would make them nearly invincible to any civilization without guns.

Skwers from Alient Planet

Skwers from Alient Planet

According to Alien Planet

Skewers create methane gas internally and combusted in four jet like pods.

P.S. The front part is lance (depicted to the right in the image), which they use as weapon.

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    $\begingroup$ This is a pretty short answer. Can you provide some details as to both how biological jet propulsion would evolve (maybe with links to your source) and how this would make the fliers 'nearly invincible?' $\endgroup$ – kingledion Apr 26 '17 at 0:09
  • $\begingroup$ They can fly higher then arrows can reach ,so they are only vulnerable when they are langing at you at almost sound speed,wich would make the formidable opponets $\endgroup$ – joao sturza Apr 26 '17 at 12:21
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I'm always amazed by how people forget/don't know about pterosaurs in such discussions about giant flying animals, only one person commented on them. Pterosaurs are the most amazing thing the earth has produced, they need more attention. There are a lot of misconceptions about them, it is pretty consensus in the field that even the biggest were capable of flight and they have biomechanical models and simulations to back it up, we have a great idea of how they did it, though the research is new - I would argue that we might understand pterosaur flight better than insect or bird flight (maybe even bats) which are much more complicated ways to fly.

I mean, before planes and pterosaurs, people thought birds were the heaviest thing capable of flight, if nobody had ever seen a flying animal I'm pretty sure they would think flight was impossible in the first place. Early naturalists often said the elephants were pushing the limits of animal size, scientists said animals couldn't survive extreme heat or pressure, etc and etc. Time and time again, those bold affirmations are proven wrong.

Yes, it is pretty possible for a flying creature to lift a human being. See that article by paleontologist Mark Witton:

https://io9.gizmodo.com/what-would-life-be-like-if-pterosaurs-were-alive-today-5909172 -> The article goes on more detail of the challenges.

Although, it would probably have to be a light human, not a heavy weight boxer or fat guy. But even so, if those animals could lift a human being, some more adaptations on top of the ones they already posses and you could lift a larger cargo, maybe his bones are some kind of very strong and light carbon based material of fiber, stronger muscles, a more efficient breathing system, maybe with large airholes like the ikrans of avatar, use of myoglobin instead of hemoglobin, etc. Small things make a huge difference.

The hardest part is not for a flying creature to lift a human being, the hardest parts are:

  • For the human to resist the coldness and rarefied air of higher altitudes, it would probably have to wear some gear or the humanoid itself would have to evolve for it, the Sherpa people of Tibet have interesting adaptations to survive high altitudes that they took from inbreeding with other ancient hominid race, it would be interesting to take a look at it;
  • For the creature to be a hexapod like you want, with four legs and two wings. That doesn't exist on earth, the closest thing I can think of are:

    1) Insects, but they have problems getting that big in the current atmospheric conditions (although if an insect developed pulmonary respiration and air sacs that's hard to predict how big they could get). The dragonflies the people of Krypton fly in Man of Steel are a good example.

    2) A wing that evolved through something like a gliding lizard. The draco volans couldn't really do it since it's wings come through the ribs bones and can't develop into another member, but there was an extinct variety called coelurosauravus who developed new bones for a wing membrane, not part of the ribs. Given enough time, evolutionary pressure and lack of competition from better flyers they could eventually develop something similar to a whole other limb used for flight. Now, this would be really problematic, since animals with extra limbs usually suffer to provide blood and energy for them, specially wings that are really energy consuming. You could get away by giving then some strange and efficient vascular and respiratory system, as I've already proposed, maybe even other auxiliary small hearts.

But, if you really want feathered wings it would be problematic since feathers can't carry as much cargo as a wing membrane. You could easily get away by saying the animal has a wing membrane that do the work and a coat of feathers above for insulation, pterosaurs probably have some simple feather-like structures that did it, and many dinosaurs developed somewhat complex feathers even though they didn't use them for flight, your animal could use them to help create turbulence or make the flight quieter or even for display, but it would be the wing membrane that would have to do all the heavy lifting - Take a look at the dinosaur Yi Chi for inspiration who also had feathers and a membranous wing.

But I don't feel like the biomechanics of flight are really the meat of your question, you wanna know what makes a good animal for domestication. CGP Gray has amazing videos about it:

https://www.youtube.com/watch?v=JEYh5WACqEk

https://www.youtube.com/watch?v=JEYh5WACqEk

You should think if the creature needs to be domesticated, it might just be tamed, which have lower requirements. On the other side, domestication could help explain how they became such good flyers, maybe they've being selective breeding and are much better flyers than their natural counterparts, I'm assuming genetic engineering to be out of question. Regardless, it would be pretty hard to tame a large flying animal. They might create them from birth and start behavioral training young. It would help if the creature was a herbivore without much to fear in the wild, and was already social with hierarchical structures and high intelligence. Maybe some imprinting could help. Needless to say they would have to train the creature tethered to the ground and use safety belts and parachutes/wingsuits for safety. Once they tamed the first ones they could use the more experienced ones to help train the new ones, so more experienced flyers could help in rescue if some accident happen, so overall I think training then might not be the hardest part.

Good luck in your world!

Edit: Came back after some thoughts, your question really got me thinking, it was the reason I've created an account in the first place. Forgot to mention other problem about wings on the back. That is that you would basically have a lot of extra weight having to carry that much members that aren't really necessary. One of the advantages of pterosaurs that made them reach such huge sizes was their quadrupedal launch that makes them use their strong wing muscles on flight and on take off (one of many, you should really research pterosaurs adaptations) and don't need such strong leg muscles. You could have a two wings, two legs animal that still walk on four like pterosaurs and have feathers.

Like that: https://in.pinterest.com/pin/765400899152037936/?nic_v1=1ahBmj9kCSbnUq%2BdkdNiWjIkSorZaTrBxBfhXYWu4hUqCZHtMpngpGTl%2FzqMoRj1n%2F

But the more important thing is to grant the creature a real ecology, and specially create a place were they have the sufficient calories and reasons to fly (like predators to escape or foods to access). The creature can develop as much crazy features as you want if it servers a real niche and it has enough calories available to do so (and don't have competition from better flyers with less crazy and more efficient body plans doing the same work). To power wings on the back in such a big creature would require enormous caloric power, maybe they co-evolved to eat big fruits that need them to disperse seeds or maybe there are some really massive flowers of a sequoia sized plant producing a ton of really high caloric nectar that they eat like a hummingbird (keeping the idea of them being herbivores and also without needing large and heavy stomachs to process leafs that are really very low in caloric density) and they're the only ones exploring that food source. Evolution doesn't mean only the most efficient animal possible survive, there are some animals that should already be extinct if it wasn't for their ability to explore a very specific food source that only they can get, sexual selection can also help explain why they retained characteristics like four limbs and feathers, animals really push the limits of evolution to please a partner. That and also selective breeding of species people culturally thought were aesthetically pleasing.

Other forms of flight like jet propulsion aren't really energy efficient, balloon creatures would have to be enormous to be able to lift their own body and a person, so they would look more like flying houses than flying mounts. Anything smaller than that and storing so much gas is much more trouble for a very little difference in their weight.

All of that of course, thinking on a very earth-like environment, any change to atmosphere density, oxygen levels and planet gravity would already make a huge difference.

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I had seen this image before, and I was amazed. This post reminded me of it, so I went looking for it. I found it in wikipedia: https://en.wikipedia.org/wiki/Pterosaur_size

enter image description here

The wikipedia page has a list of large Pterosaurs and a small discussion about their flight capabilities. It could serve as a starting point for more research.

EDIT: wikipedia also says that they were probably quadripedal.

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John Norman in his Gor novels envisions birds flown by humans for both combat and transport.

Anne McCaffrey talks about dragons, tamed for humans to ride in the Dragonriders of Pern series.

Both work great in stories. In reality of course, even if the flying creature can carry the weight of a human being (and his saddle and cargo) on its back, how do you explain the animal evolving to have that capability? Birds on earth just don't have the physical strength to do anything similar in scale, their spines can't handle the strain. And even though some can carry a weight that would (relative to their own) be high enough in their feet, they typically don't have a lot of endurance doing so, severely limiting the usefulness of using them to carry humans.

Of course in a fantasy setting, you don't really have to explain such things to your readers, but it helps if it's somewhat believable (JN pretty much hints at selective breeding and the lower gravity of his planet, for example).

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You could take any of the previous answers and slightly change the structure:

  • Cartilage or hollow bones are lighter
  • Instinct used rather than intelligence (less heavy brain to carry)
  • Stripped-down digestive system
  • Thinner skin
  • Smaller skull
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