Re’em (a wild buffalo) in the Hebrew Bible as a metaphor representing strength (because of his corns) was later translated as monokeros in Greek Septuagint and as unicornis in Latin Vulgata. The translators simply associated the writings of Greek naturalists about this fabulous animal with the Hebrew Re’em. That was the way the unicorn was introduced to the Christian symbolism.
During the Middle Ages the image of unicorn was taken from ancient and biblical sources. This animal was presented as a horse, a goat or an ass. In Physiologus there is an explanation that a unicorn can be trapped only by a Virgin. The Virgin represents Virgin Mary and the hole allegory stands for the Incarnation. When unicorn sees her he puts his head on her lap and falls asleep. Then the hunters can take him. This tradition was transmitted to the later medieval Bestiary. In some versions the unicorn is directly associated to the Christ: one corn symbolizes the unity of Son and Father.
Unicorn symbolizes also the divine revelation, the penetration of divine to the creature. In the Christian iconography it represents the Virgin fecundated by the Saint Spirit. The horn was compared with the psychical phallus, the symbol of spiritual fecundity. In the Middle Ages it became a symbol of the incarnation of the Word of God in the womb of Virgin Mary.
Until the 17th century unicorns were believed as real animals and their horns could be seen and possessed. They became very valuable because of their purifying powers. It was believed that they cured diseases, neutralized poisons and cleaned water.