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This is a part of the Anatomically Correct series.

If a magic-free fantasy/SF world were to have naturally evolved hexapods as humanoid as possible, living among bipedal humanoids, how would they actually look, live and behave like?

  • Fur, feathers or naked skin with some hair (pubic, head/mane, chest, tail)? Clothes and armor?
  • Feet with (even/odd) toes, (split) hooves or paws with claws? Knees?
  • Short or long, fancy or usable or stabilizing, bushy or skinny tail?
  • Head and brain size, also other organs (e.g. for breathing)?
  • Horns or antlers? – Cows and deers are roughly the same size as mules, ponies and horses, but this is also of interest for 2-legged goat and cattle chimeras (i.e. faun and minotaur).
  • Movable flat long hairy ears?
  • Grass, leaves, fruit, meat or mixed diet (hence teeth and mouth)?
  • Size, capabilities and developmental status at birth?
  • Age and duration of infancy, childhood, puberty, adolescence, adulthood and life expectancy?

There are several six-limbed mythical creatures (hexapods) with human-like intelligence or at least typical features of mammals, e.g. quadrupedal centaurs or winged angels, dragons and griffons, maybe also harpies in Mediterranean/European mythology. Like other fantastical creatures, e.g. sphinx, minotaur and faun, they are mostly described and depicted as blends (chimeras) of known animals and usually a human part, but that doesn’t have to be taken literally. For an example of an unusual, because very human-like centaur, see this NSFW picture. Shoops like that are probably easy to make for savvy people, but so are “humanimal” morphs. They may be aesthetically pleasing, but not very realistic.

There are no hexapod mammals in reality obviously, whereas 6 (and more) equal legs (or tentacles etc.) are normal in insects and some water-based animals (crustaceans, octopuses, squids) and some have developed further their back limbs (e.g. grasshopper, crickets) or front limbs (e.g. mantis) or at least one of them (e.g. some crabs).

You cannot simply put a human torso onto a horse body to get a centaur as several questions here have shown, so I think of horse-like as meaning “the approximate size and number of legs of a horse” and of human-like as “able to think and converse and use manual tools”.

From the nipples question, I assume 2 teats at the frontal crotch or slightly above. The genitals are probably at the rear end, as is the anus. Grazing may be possible, but cannot be the main source of nutrition; an omnivore diet and digestive system seems plausible although not necessary. I like the navel nostril idea mentioned in an answer to the oxygen question, but anything else would look too strange, so probably big noses or big heads over all and probably a short muscular and flexible torso.

Wings are hard to manage for humanoid bodies as they are and I think even more so if separate from arms/hands, so I would rule out winged hexapods altogether.

It’s not at all realistic for centaurs etc. to be able to interbreed with humans or the animals they’re looking similar to, but interspecies sex (or rape) is probably inevitable and the topic of much folklore.

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  • $\begingroup$ I cannot believe this hasn't been asked before. Also thanks for linking to my questions $\endgroup$
    – TrEs-2b
    Oct 16, 2015 at 18:21
  • $\begingroup$ I think the only way for hexapodal vertebrates to exist is to make the salamander of mythology a reality. $\endgroup$
    – JohnWDailey
    Oct 21, 2015 at 22:51
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    $\begingroup$ This should have been rolled into the Anatomically Correct series many moons ago. Asking for forgiveness rather than permission, I'm going to make some minor edits to identify its place on that august roll and include an appropriate link on the roll. $\endgroup$
    – JBH
    Jan 14, 2019 at 19:07

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I think that one can plausibly suppose that sentient tool-users could evolve from hexapods, with the species evolving the ability to walk on just four legs with the front pair of limbs devoted to manipulation in a way parallel to the way humans evolved the ability to walk on just two legs with what were once front legs becoming arms.

The most economical explanation for why on a given planet hexapods rather than quadrupeds should win the sentience lottery is that the planet has higher gravity than earth. (Obviously this makes the idea of them living among bipedal humanoids, and certainly of bipedal humanoids living among them, much less likely.) Higher gravity would favour creatures that were lower to the ground and more stable than bipeds. Their legs would be shorter and stumpier than horse legs. They probably would have knees, but some flexibility would have to be sacrificed to get more strength in a manner more like elephant knees than horse or human knees.

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    $\begingroup$ The "torso" could also have developed from a long neck. Giraffe Centaurs. $\endgroup$
    – TheSexyMenhir
    Oct 16, 2015 at 14:48
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    $\begingroup$ I wonder whether you need high G to bias evolution. The fish which crawled out of the water to give rise to amphibians, reptiles etc. had four fins, with the result that all derived life lacks the genes necessary to develop more than four limbs (and a tail). Had that fish had six fins, perhaps centauroids (and birds with small hands) might be the norm. More speculatively, intelligence (driven by the ability to use tools) might have faced far less of an uphill struggle to evolve. $\endgroup$
    – nigel222
    Oct 20, 2015 at 16:18
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    $\begingroup$ @nigel222, I agree that whether the first fishy-type creature to crawl out of the water has six fins or four is pure chance. On Earth it was four, so that cut the centaur possibility out from the start. But even if the original fish-out-of-water had six fins there is a cost in brain processing power needed to control more legs. (See this WBSE answer worldbuilding.stackexchange.com/questions/569/…) But this could be traded against the advantage in stability, greater in high gravity, of the centaur form. $\endgroup$
    – Lostinfrance
    Oct 20, 2015 at 17:22
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    $\begingroup$ tbh, I don't know fish (paleo or present) that had six lateral fins. They all have four, so rest of fins are dorsal/anal/caudal.... $\endgroup$
    – Swift
    May 7, 2018 at 13:26
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    $\begingroup$ It's marginally similar to scorpions adapting sensory organs on the head into graspers $\endgroup$
    – user73215
    Apr 10, 2020 at 22:22

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