A unicorn (from Latin unus 'one' and cornu 'horn') is a mythological creature. Though the modern popular image of the unicorn is sometimes that of a horse differing only in the horn on its forehead, the traditional unicorn also has a billy-goat beard, a lion's tail, and cloven hooves—these distinguish it from a horse. Marianna Mayer has observed (The Unicorn and the Lake), "The unicorn is the only fabulous beast that does not seem to have been conceived out of human fears. In even the earliest references he is fierce yet good, selfless yet solitary, but always mysteriously beautiful. He could be captured only by unfair means, and his single horn was said to neutralize poison."

Unicorns in antiquity

A one-horned animal (which may be just a bull in profile) is found on some seals from the Indus Valley Civilization. Seals with such a design are thought to be a mark of high social rank.

The translators of the Authorized King James Version of the Bible (1611) employed unicorn to translate re'em, providing a recognizable animal that was proverbial for its un-tamable nature.

: Will the unicorn be willing to serve thee, or abide by thy crib? Canst thou bind the unicorn with band in the furrow? or will he harrow the valleys after thee? Wilt thou trust him, because his strength is great? or wilt thou leave thy labour to him? Wilt thou believe him, that he will bring home thy seed, and gather it into thy barn?

: He maketh them also to skip like a calf; Lebanon and Sirion like a young unicorn.

: ...he hath as it were the strength of a unicorn

Unicorns are not found in Greek mythology, but rather in accounts of natural history, for Greek writers of natural history were convinced of the reality of the unicorn, which they located in India, a distant and fabulous realm for them. The earliest description is from Ctesias who described them as wild asses, fleet of foot, having a horn a cubit and a half in length and colored white, red and black. (quoted by Photius) Aristotle must be following Ctesias when he mentions two one-horned animals, the oryx (a kind of antelope) and the so-called "Indian ass". Strabo says that in the Caucasus there were one-horned horses with stag-like heads. Pliny the Elder mentions the oryx and an Indian ox (perhaps a rhinoceros) as one-horned beasts, as well as "a very fierce animal called the monoceros which has the head of the stag, the feet of the elephant, and the tail of the boar, while the rest of the body is like that of the horse; it makes a deep lowing noise, and has a single black horn, which projects from the middle of its forehead, two cubits in length." In De natura animalium, Aelian, quoting Ctesias, adds that India produces also a one-horned horse (iii. 41; iv. 52), and says (xvi. 20) that the monoceros was sometimes called cartazonon, which may be a form of the Arabic karkadann, meaning "rhinoceros".