Kurotowa
Legend
Or, it turns out, very often entirely real and well known animals. Just with the descriptions filtered through an international game of Telephone in an era without photographs or detailed zoological understanding.Yeah, though sometimes they're elaborate historical hoaxes! And I imagine that many of them were from people finding skeletons of things they couldn't identify, such as the elephant-skull cyclops idea or, frankly, dinosaurs.
The kirin was a giraffe. Unicorns were, duh, rhinoceroses. Behemoth was a hippopotamus and leviathan was a crocodile. Basilisks, a snake with a crown and venom so deadly it could kill with a glance, were cobras.