The unicorn is one of the few mythical creatures whose origin can be traced almost step by step, and the steps are textual rather than mythological.
Ctesias, a Greek physician who spent years at the Persian court and wrote around 400 BCE, describes in his Indica a wild ass of India, white-bodied, with a horn on its forehead, whose horn made into a drinking vessel protects against poison. Ctesias never went to India and was relaying report; the animal behind the description is most plausibly the Indian rhinoceros, possibly blended with accounts of oryx or of Tibetan wild ass. Aristotle mentions one-horned animals, and Pliny repeats and elaborates, giving the monoceros a stag's head, elephant's feet and a black horn.
The decisive step is a translation error. The Hebrew Bible uses the word re'em for a wild bovine, an animal of great strength, the aurochs, now extinct. The translators of the Septuagint, in Alexandria in the third and second centuries BCE, rendered it monokeros, one-horn. Jerome's Latin Vulgate followed with unicornis and rinoceros. So passages such as Numbers 23:22 and Job 39:9-10, which describe an untameable wild ox, became in Greek, Latin and then English descriptions of a unicorn. The King James Version has unicorns in nine places, and the animal thereby acquired scriptural authority it never had in Hebrew.
That authority is the whole reason the unicorn was treated in medieval Europe as a real animal rather than a fable. It was in the Bible. Physicians, bestiarists and apothecaries proceeded accordingly, and alicorn, powdered unicorn horn, in practice narwhal tusk, was sold at extraordinary prices as an antidote to poison well into the seventeenth century.