Readin' the encyclopedia

The unicorn appeared in early Mesopotamian artworks, and it also was referred to in the ancient myths of India and China. The earliest description in Greek literature of a single-horned (Greek monokerōs, Latin unicornis) animal was by the historian Ctesias (c. 400 bce), who related that the Indian wild ass was the size of a horse, with a white body, purple head, and blue eyes, and on its forehead was a cubit-long horn coloured red at the pointed tip, black in the middle, and white at the base. Those who drank from its horn were thought to be protected from stomach trouble, epilepsy, and poison.

-Encyclopaedia Britannica

Things that don't sound like rhinos for 500, Alex.