I'm building a whole new world for a table top RPG so I don't want to completely alienate my players from the world. But if I want to include horses or whales, do I need to reinvent the wheel and make a "horse-like" creature instead? What are the drawbacks?
"Kalgash is an alien world and it is not our intention to have you think that it is identical to Earth, even though we depict its people as speaking a language that you can understand, and using terms that are familiar to you. Those words should be understood as mere equivalents of alien terms-that is, a conventional set of equivalents of the same sort that a writer of novels uses when he has foreign characters speaking with each other in their own language but nevertheless transcribes their words in the language of the reader. So when the people of Kalgash speak of "miles," or "hands," or "cars," or "computers," they mean their own units of distance, their own grasping-organs, their own ground-transportation devices, their own information-processing machines, etc. The computers used on Kalgash are not necessarily compatible with the ones used in New York or London or Stockholm, and the "mile" that we use in this book is not necessarily the American unit of 5,280 feet. But it seemed simpler and more desirable to use these familiar terms in describing events on this wholly alien world than it would have been to invent a long series of wholly Kalgashian terms.
"In other words, we could have told you that one of our characters paused to strap on his quonglishes before setting out on a walk of seven vorks along the main gleebish of his native znoob, and everything might have seemed ever so much more thoroughly alien. But it would also have been ever so much more difficult to make sense out of what we were saying, and that did not seem useful. The essence of this story doesn't lie in the quantity of bizarre terms we might have invented; it lies, rather, in the reaction of a group of people somewhat like ourselves, living on a world that is somewhat like ours in all but one highly significant detail, as they react to a challenging situation that is completely different from anything the people of Earth have ever had to deal with. Under the circumstances, it seemed to us better to tell you that someone put on his hiking boots before setting out on a seven-mile walk than to clutter the book with quonglishes, vorks, and gleebishes.
"If you prefer, you can imagine that the text reads "vorks" wherever it says "miles," "gliizbiiz" wherever it says "hours," and "sleshtraps" where it says "eyes." Or you can make up your own terms. Vorks or miles, it will make no difference when the Stars come out.
-- Isaac Asimov's introduction to Nightfall (with Robert Silverberg), 1990
Well, you're not only building a world, you're also building a game, and this changes everything.
If you want an immersive experience with a world that feels different, brand new creatures are a must. They can be "inspired" from earth creatures, but in the end this won't be a "we changed the skins but they are the same" output, as every change will have cultural and practical results. But it will be awesome.
On the other hand, if you don't want to have to explain everything all the time, using earth-like creatures will both save you an insane amount of brain-time and will help your players to better imagine what you're speaking about.
Remember, this isn't a novel, it's a shared fantasy with your players. With a role playing game, it's generally accepted that the endgame isn't the worldbuilding or the balance within the rules, but the fun itself. So... this world you're building, in the end it's a tool to improve on this. To have fun.
If your game is about immersion and you (and your players) don't mind the descriptions, full creation is great. Sometimes, a horse can be much more than a horse.
If your game is about actions, you can use archetypes and hit the ground running! After all, your players know what the concept of a horse is, so they can hit the ground running. If you do it this way, don't change the names, as it would be counter-productive for you to create a world with easy-to-understand concepts... and then make sure nobody gets them at a glance. These are tools to your game. Use them properly.
You can go halfway, too. To introduce a "horse-like creature" is a good way to minimize descriptions and explanations while keeping a door open on exotic creatures and cultures. You have to be more careful, though, because your players might assume some things about these earth-like creatures, and if you see things differently it will fudge their plans and lesser the overall experience. This is still the way most authors and developers go.
Whatever you do, in the end, always remember this this is for fun. Yours and you players.
The technical term for this is 'calling a rabbit a smeerp'. In other words, taking something which exists on Earth and give it an alien name just for show. The rule of thumb when writing is to only call alien things by alien names when they are functionally distinguished from the Earth counterparts. Not merely different, but perform a functionally different role.
So, say a normal rabbit - not a smeerp. A blue rabbit - a bit better, but not exactly a smeerp. Maybe drop a line about it. A blue rabbit which has poisonous fangs and can see in the dark - smeerp!
Ursula K. Le Guin (tried to find the original text from her but didn't) wrote something about neologisms. It was about tech (not creatures). She said that if something is well known, use it (don't rename it). That sci-fi is a bigger thing than your story, a growing body, where things are created and live.
So for creatures, I usually use the same reasoning. If it's a horse, I call it horse. If it's a horse with wings, I call it a winged-horse or a flying horse. I also use general expressions from biology, like quadruped and herbivore for alien fauna, instead of making up new terms.
If you look up scientific names, classes, expressions from earth's biology they are actually very surprising and alien. They can be a great source of inspiration.
Vary the familiar within the landscape of the known.
If you make everything weird and invented, it can feel contrived. Readers / players have to learn everything new.
If you have a "strange new world" but your characters are basically cowboys, it feels lazy.
I propose you vary the familiar within the realm of the known. Examples:
In the movie Princess Mononoke, Prince Ashitaka rides a red elk. There were horses in the movie, but having him ride an elk set him apart from the people around him. An elk is not that weird. For its part in the story his elk could almost have been a horse - a large quadruped, loyal, herbivorous. But when it did things like jump very far that could be explained by the fact that is was an elk.
- In the book Hiero's Journey (an inspiration for many far future post apocalyptic SF scenarios), Hiero rides a psychic moose. It is horselike for purposes of being a mount but it has some other abilities. Fun stuff and also not super outré because most folks are familiar with the starting point of "moose". Plus mooses are great.
If horses seem boring but you don't want the place your characters sit to be the focal point of novelty and innovation, use a real nonhorse animal for your mounts. Choose according to the ecosystem your players will traverse. Remember that people rode wagons and then chariots before they rode atop horses. There are more animals that could pull a chariot than could be ridden with a saddle. Chariots are not weird inventions but neither are they what cowboys and knights ride.
I once spoke with a zookeeper at the San Diego who clearly had a rapport with the giraffes. I asked if he thought they would pull him in a chariot and he got very happy speculating about how that might be done - clearly an idea that sparked his imagination.
Re whales - whales are pretty spectacular as is. An omnivorous whale would be interesting for an RPG. Seeing a blue whale rip into a carcass would be pretty alarming. There are lots of other real creatures which could or did fill the niche of whales in the history of the world. Or you could use sea monsters - Heuvelmans categorzied them in his book In the Wake of the Sea Serpents and you could borrow some of his. https://mythology.wikia.org/wiki/Sea_serpent
My rule of thumb is that if it fulfils a similar function or fills a similar niche then call it by the name but you can point out the differences. Above all keep it simple. It can be saddled and ridden then why not just call it a horse? It may have scales and antennae but it fulfils the function of a horse, just refer to it first time as a scaly-reptilian horse and thereafter just a horse.
In-game you may need to specify the horse in detail for encounters, combat etc. and deal with scaly armour or night vision but then in any normal fantasy RPG a horse needs to be specified as there is a huge difference between a Hobbit's pack pony and the furious ton of meat, muscle and armour that is a Paladin's war horse.
If you create a whole new creature then by all means name it or if you create an animal that either does not have an earth parallel or else has too many parallels then name it . For example if you have a long thin furry legless creature that lives under ground is it an elongated legless ferret? A furry snake-mole?
The only other thing I would suggest is that if this is a new world colonised by immigrants (like the US or Australia) then the immigrants tend to bring with then names from the old country. Many 'New World' places were named after the hometown of the settlers, similarly many animals were named for similar looking animals from home. There is a robin in the US named after the European robin just because it looks similar-ish. In your world it is plausible, if not expected, for the incomers to name a horse-like creature a horse.
I don't know if this would be annoying for the players, but I would use different spelling and unicode characters to slightly alter the spelling of the word, make it recognizable and readable, but alien enough that in the mind of the player/reader it can easily be some other creature
a rabbit could be a rābeet
a horse would be a hőƦsƎ
a cat could be a katt
a dog would be a Ɖǒǥ
So you would choose a font that has all of these characters, in a way making your own alphabet used only for creatures, places, general objects (maybe you also have some special swoūrd that has a really weird shape, but cuts like a sword), but not for regular story telling.
You can take the example video games set (since you're also building your world for a game). Basically, in video games, most worlds are just like Earth, until they're not. What does that mean? They use the Earth template as long as it suits them, whenever this template needs editing (new creatures, new deities, new races, etc) they do just that. For example, The Witcher series added a lot of creatures, while still keeping some that are familiar to us. Everything else is pretty similar to medieval Earth. The Elder Scrolls series added magic and deities, while still using the medieval Earth template. Also, they can remove items from the template that don't suit them, just like the Fallout series did with the transistor. I think this is what you should do with your world, build it like Earth, and alter it in ways that suit your objectives.