Isaac Asimov's Worlds of Fantasy. Book 6: Mythical Beasties

Unicorn

The unicorn is a legendary one-horned creature. Almost all horned animals have two horns, one on each side of the head. An exception is the rhinoceros, which has a single horn on the nose, and there is some thought that the legend of the unicorn in the Middle East and in Europe originated with vague tales of rhinoceroses in India.

Much more likely is thai it originated with Assyrian representations of wild bulls in profile. In profile, naturally, the two horns overlapped and only one was visible.

Somehow this gave rise to the thought of one-horned bulls.

In the Bible, mention is made of a creature named re-em in Hebrew. The reference was to the wild bull (called an aurochs in Europe) which is the primitive ancestor of domestic cattle, and which is now extinct. It was described as immensely powerful.

In the Greek translation of the Bible, re-em was translated as monokeros ("one-horn"), the reference being to the Assyrian representations in profile. This became unicomis (also "one-horn") in Latin, and the legend was fixed.

Somehow, the unicorn tale became preltied up. Instead of a one-horned bull remarkable for its strength, it became a one-horned horse, or horselike antelope, remarkable for its beauty and swiftness. It began to symbolize purity and virginity; it could be caught only by a virgin woman.

Its horn is now pictured as a tight spiral, extending forward and slowly tapering. This is actually the "horn" (really, the tooth) of the narwhal, a species of whale. The narwhal tooth was brought back by sailors and sold as a unicorn horn for large sums, since such a horn was considered a powerful aphrodisiac.

The following story deals with the unicorn at its prettiest.