This isn’t because Greeks never had seen a person riding a horse before
Actually, this was more uncommon than you think. Horses during the ancient period were too small and weak to be ridden, at least in the area around greek and the Mediterranean sea general. Thats why everyone used chariots instead of cavalry.
Some classic origins of mythical creatures: The source for unicorns were people describing rhinos (although the horns of Narwhales were also mistaken for unicorn horns. They have such a "unicorn horn" on display in Vienna, next to the Holy Grail).
Mermaids probably come from the description of manatee.
In china there are actually snakes that jump and glide from tree to tree which could have been the source for eastern dragons.
I will skip the silliness about ancient horses since it was corrected previously. . .Nor will I discuss the incredibly limited and naïve medieval misconceptions, because as much as I love them, the scholars of that period were dangerously credulous, woefully inexperienced, and sadly very easily mislead. They did the best they could with what they had, but the fall of Rome left them isolated and with only incomplete sources from which to draw. However, you are absolutely wrong about the Rhino. Pliny the Elder described Rhinos with sufficient accuracy that there is little doubt that he was personally familiar with the animal, Philemon Holland's translation of Pliny's unicorn however is as follows:
"“But the most fell and furious beast of all other, is the Licorne or Monoceros: his bodie resembleth an horse, his head a stagge, his feet an Elephant, his taile a bore; he loweth after an hideous manner; one blacke horn he hath in the mids of his forehead, bearing out two cubits in length: by report, this wild beast cannot possibly be caught alive."
Now Rhinos can be aggressive, but they are hardly furious, and also noone would compare the body of a Rhino with a horse. Also, even the Black Horn Rhino's horn is mottled grey and black -just like its body. Describing the horn as black suggests it was a unique feature. Also, the Romans routinely caught and employed Rhinos in the colosseums. We know, for instance, that in the inaugural games of the Flavian Amphitheatre Rhinos were pitted against both bulls and elephants and men and are depicted in numerous fresco. So yeah, the unicorn was never a Rhino, that was a conflation which would arise later. That's cool though, because noone really knows what the fuck Pliny was talking about either, so don't feel bad. I tend to believe he was describing this though:
Elasmotherium sibericum
![[Linked Image]](https://www.popsci.com/resizer/DXEH16XkL_76q9kB8ApWVMQvmV4=/851x500/arc-anglerfish-arc2-prod-bonnier.s3.amazonaws.com/public/YW52HBVVVFWBB3IDOY7D2UOHAE.jpg)
Not an actualu specimum. but for scale:
![[Linked Image]](https://i.pinimg.com/originals/c0/40/33/c040338a249befa9946fc76cfc82ec40.jpg)
The only problem is that we have no corroborative fossil evidence, our best guess is that it went extinct from 25,000 years before Pliny's account. A small detail