Kamikaze Midget said:
I always thought the unicorn myth came from the one-horned ibix...I mean....I may be no great mythographer, but something that big with a horn on it's nose can't become something quite a bit different with a horn on it's head, can it?
On second thought, I wouldn't be surprised...but...eep.
Sure it can.
You've gotta remember, first, a lot of these creatures weren't seen clearly or close up. Second, even when they were seen, people often didn't have anyting but their old paradigm with which to express what they saw. Third, in would be quite some time before news of, say, a rhino got back home. In that time, it might have passed through several people and changed in the telling, and even if you were getting an eyewitness account, it was probably quite some time since he saw the critter, and we all know how reliable memory is.
As far as other real world sources, one of the many sources of the vampire myth is a condition--I forget what causes it--when the blood of a corpse gathers in the facial area. This causes red, life-like cheeks, and sometimes a trickle of blood at the corners of the mouth.
(Incidentally, the name "hippopotamus" is from the Greek for "water horse," and there's not much horse-like about them. I just mention it as an example of this sort of thing.)