I've recommended this one before, but for all the non-human vertebrate likers out there... the art of animal drawing

Instead all those creatures you know came about from transcription and translation errors from copying Greco-Roman sources (who themselves got them from travelers’ tales from Persia and India - rhino -> unicorn, tiger -> manticore, python -> dragon, and so on).

So unicorns are real

behold… a unicorn

I always thought animals in medieval manuscripts looked like the result of having to draw say. A Tree Kangaroo, but your only source for what it looked like was your friend who heard it from a fellow who knows a man who swears he saw one once, whilst very drunk and lost, and I am SO PLEASED  to find out this is, in fact, the case.

Questing Beast

- Neck of a snake

- body of a leopard

- haunches of a lion

- feet off a hart (deer)

So is it

Or….

don’t forget that some of the legendary creatures they were describing were from other people’s mythos which were passed down in the oral tradition for gods know how long. You know what existed in Eurasia right around the time we were domesticating wolves into dogs?

these beasties. For a long time, science had them down as going extinct 200 thousand years ago, but then we found some bones from 36 thousand years ago. Which, y’know, is quite a difference. Since you can bet that any skeleton we find is not literally the last one of its kind to live, many creatures have date ranges unknowably far outside the evidence.

In South Asia there were cultures that described a man-beast/troll forrest giant  who’s knuckles dragged the ground, and everybody from the west was sure it was superstitious mumbo jumbo, but you know what used to live there?

And did you know that some of the earliest white colonizers of the Americas heard accounts that there were natives still alive who had seen and hunted and eaten a great hairy beast, shaggy like the buffalo but much bigger, with a long thin nose like a snake and two giant fangs… so, like, mammoths, you know? but they were totally discounted because europeans of the time were like, elephants live in Africa and aren’t hairy, you can’t fool us, pranksters!

Anyway, the point is between the early writing game of telephone description thing talked about by OP, and the discounting of native cultural accuracy, I’m pretty sure most legendary creatures are in fact real animals one way or another 

It can’t explain every single legendary creature, but yes, this is super important. Because History relies on written sources, it tends to sweep oral tradition under the rug, even if there’s a lot of interesting informations in it.

And it’s not just living animals that were badly described, or which descriptions got exaggerated over the course of centuries or through translation errors. Sometimes, people finding fossil bones of extinct animals might have also influenced some myths!

By now this is pretty well-known but it has been theorised that the Greek myth of the cyclops was started when people found Deinotherium skulls. Now you might say, uh, how is it possible to think a cousin of the elephant is a huge human dude with one eye?

Well-

- the big nasal opening kinda looks like an eye if you have no idea what kind of animal had this kind of skull (you can read more about this theory in this old National Geographic article if you like).

Here’s a less well-known one; the griffin is a mythological hybrid with the body of a lion and the head and wings of an eagle. The earliest traces of this myth come from ancient Iranian and ancient Egyptian art, from more than 3000 BC. In Iranian mythology, it’s called شیردال‌ (shirdal, “lion eagle”). Now, it’s been the subject of some debate and it’s not confirmed, but there’s a theory that people might have seen some Protoceratops and Psittacosaurus fossils in Asia and might have interpreted it as “a lion with an eagle’s head”:

Check the “origin” part of the wikipedia page for “griffin” if you want to find more sources for this theory and for the arguments against it! Again, it’s just a theory, but I think it’s super cool.

This is a pretty well accepted theory for why dragons (or animals we group as like dragons, eg wyverns and drakes) are seen in mythos almost worldwide - because people found dinosaur bones, looked at them, and went “oh fuck what’s that? some big…. lizardy thing?” and then created dragons.

Also many deagon legends are simply exaggerations of well-known living reptiles like snakes and crocodilians.a

It also explains why dragons can look so different in the myths of the various regions.

In asia, Dragons tend to look very long and snake like:

One of the most common dinosaurs that used to like in the asia region, so would have been the most common fossils found by people:

The Mamenchisaurus, this thing is just all neck and tail! You find just half a fossilised skeleton of this monster, you can easily end up thinking of a long snake-like beast.

South America also has legends snake-like dragons among some of its peoples:

What fossils from pre-historic south America could be found?

The Titanoboa, which can easily grow to be 40 feet long.

In North America there is the Piasa Bird

Which wikipedia tells me comes from “ the large Mississippian culture city of Cahokia,” it’s describes as

What fossils could have been found in that region:

Pterosaur, and Triceratops. Features of both sets of skeletons could have been merged into one legendary creature.

Then we get our European style dragon:

One of the most common fossils that could have been found was a Cetiosaurus 

which, despite being a herbivore, looked to have a mouth of sharp looking teeth, consistant with a dragons.

Dragons amongst the peoples of Africa are even more varied, but most revolve around some kind of giant snake-like creature. As a quick example, we’ll take Dan Ayido Hwedo commonly found in West African mythology.

Fossils in that area could have been included the Aegyptosaurus:

A quick google search tells me that most Sauropods: well known for being long necked and long tailed, are found in Africa.

If you found only a half complete skeleton of this thing; which is likely, because it’s rare to find a complete dinosaur skeleton, you could easily think of a giant snake monster.