The unicorn is a mythical beast, a symbol of supreme power. It is like a horse, but with cloven hooves and a long, single horn protruding from the middle of its forehead. The horn is reminiscent of the twisted tusk of a narwhal, and is made of ivory. The unicorn is sometimes depicted with a goat-like beard and sometimes with the hind legs of an antelope and the tail of a lion.

Ctesias, the fourth century BCE Greek historian, gives the earliest description of a unicorn. He states that in India there were wild white asses with a horn a cubit and a half in length. These beasts were coloured white, with blues eyes and red heads, and their horns were used to make drinking vessels that protected against poison. Aristotle also mentions the unicorn, as does Pliny. This belief in the efficacy against poison endured until the time of Charles II of England, when cups of rhinoceros horn were engraved with pictures of unicorns. In France, as late as 1789, instruments of "unicorn horn" were used to test royal food for poison.

The unicorn is found in the religion and sacred writings of Persia, in which it is described as having only one horn. The horn is spoken of as being hollow and made of pure gold.

Many Buddhist temples contain paintings and sculptures of unicorns and from Tibet the unicorn is thought to have spread to China, where it is known as the ki-lin. Ki means "male" and lin means "female." The unicorn is indeed thought to be both male and female, equated with hermaphrodite in alchemy and rich in the symbolism of opposites.

The alchemist Lambsprinck's De Lapide Philsophico (Frankfurt, 1625) depicts a stag facing a unicorn. The text says: "A deer and a unicorn are hidden in the forest. Blessed can we call the man who artfully can capture and tame them." The forest is the body; the unicorn, the spirit (sulphur, male); the deer is the soul (mercury, female).

Stories of unicorns flourished in medieval times. A sixteenth-century tapestry depicts a young woman holding a mirror and petting a unicorn. The belief was that only a young virgin was innocent enough to capture the beast. Seated alone, in a forest under a tree, the unicorn would come to her and place its head in her lap. There it would permit her to caress it to sleep.