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. 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 On the Nature of Animals (Περὶ Ζῴων Ἰδιότητος, 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 (Greek: μονόκερως) was sometimes called cartazonos (Greek: καρτάζωνος), which may be a form of the Arabic karkadann, meaning "rhinoceros". Cosmas Indicopleustes, a merchant of Alexandria, who lived in the 6th century, and made a voyage to India, and subsequently wrote works on cosmography, gives a figure of the unicorn, not, as he says, from actual sight of it, but reproduced from four figures of it in brass contained in the palace of the King of Ethiopia. He states, from report, that "it is impossible to take this ferocious beast alive; and that all its strength lies in its horn. When it finds itself pursued and in danger of capture, it throws itself from a precipice, and turns so aptly in falling, that it receives all the shock upon the horn, and so escapes safe and sound."

Medieval Unicorns Medieval knowledge of the fabulous beast stemmed from biblical and ancient sources, and the creature was variously represented as a kind of wild ass, goat, or horse.

The predecessor of the medieval bestiary, compiled in Late Antiquity and known as Physiologus (Φυσιολόγος), popularized an elaborate allegory in which a unicorn, trapped by a maiden (representing the Virgin Mary), stood for the Incarnation. As soon as the unicorn sees her, it lays its head on her lap and falls asleep. This became a basic emblematic tag that underlies medieval notions of the unicorn, justifying its appearance in every form of religious art. Interpretations of the unicorn myth focus on the medieval lore of beguiled lovers, whereas some religious writers interpret the unicorn and its death as the Passion of Christ. The myths refer to a beast with one horn that can only be tamed by a virgin; subsequently, some writers translated this into an allegory for Christ's relationship with the Virgin Mary.

The unicorn also figured in courtly terms: for some 13th century French authors such as Thibaut of Champagne and Richard de Fournival, the lover is attracted to his lady as the unicorn is to the virgin. With the rise of humanism, the unicorn also acquired more orthodox secular meanings, emblematic of chaste love and faithful marriage. It plays this role in Petrarch's Triumph of Chastity. The royal throne of Denmark was made of "unicorn horns" – almost certainly the horns of the Narwhal. The same material was used for ceremonial cups because the unicorn's horn continued to be believed to neutralize poison, following classical authors.

The unicorn, tamable only by a virgin woman, was well established in medieval lore by the time Marco Polo described them as scarcely smaller than elephants. They have the hair of a buffalo and feet like an elephant's. They have a single large black horn in the middle of the forehead... They have a head like a wild boar's… They spend their time by preference wallowing in mud and slime. They are very ugly brutes to look at. They are not at all such as we describe them when we relate that they let themselves be captured by virgins, but clean contrary to our notions.

It is clear that Marco Polo was describing a rhinoceros. In German, since the 16th century, Einhorn ("one-horn") has become a descriptor of the various species of rhinoceros. The ancient Norwegians were said to believe the narwhal to have affirmed the existence of the unicorn. The unicorn horn was believed to stem from the narwhal tooth, which grows outward and projects from its upper jaw. In popular belief, examined wittily and at length in the seventeenth century by Sir Thomas Browne in his Pseudodoxia Epidemica, unicorn horns could neutralize poisons. Therefore, people who feared poisoning sometimes drank from goblets made of "unicorn horn". Alleged aphrodisiac qualities and other purported medicinal virtues also drove up the cost of "unicorn" products such as milk, hide, and offal. Unicorns were also said to be able to determine whether or not a woman was a virgin; in some tales, they could only be mounted by virgins.

Origins Hunts for an actual animal as the basis of the unicorn myth, accepting the conception of writers in Antiquity that it really existed somewhere at the edge of the known earth, have added a further layer of mythologizing about the unicorn. These have taken various forms, interpreted in a scientific, rather than a wonder-filled manner, to accord with modern perceptions of reality.

Alleged evidence Among numerous finds of prehistoric bones found at Unicorn Cave in Germany's Harz Mountains, some were selected and reconstructed by the mayor of Magdeburg, Otto Von Guericke, as a unicorn in 1663. Guericke's so-called unicorn had only two legs, and was constructed from fossil bones of a Woolly rhinoceros and a mammoth, with the horn of a narwhal. The skeleton was examined by Gottfried Leibniz, who had previously doubted the existence of the unicorn, but was convinced by it. Baron Georges Cuvier maintained that, as the unicorn was cloven-hoofed, it must therefore have a cloven skull (making the growth of a single horn impossible); as if to disprove this, Dr. W. Franklin Dove, a University of Maine professor, artificially fused the horn buds of a calf together, creating the external appearance of a one-horned bull

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