The word "unicorn" is based on the Hebrew word re'em ("horn"), in early versions of the Old Testament translated as "monokeros", meaning "one horn", which became "unicorn" in English. The creature is possibly based on the rhinoceros or the narwhal, a marine creature with one horn.
In the west it was first mentioned by the Greek historian Ctesias in 398 BCE. According to him they lived in India and he described them as 'wild asses which are as big as a horse, even bigger. Their bodies are white, their heads dark red and their eyes are deep blue. They have a single horn on their forehead which is approximately half-a-meter long.' This description was based on the tales of travelers, and is a mixture of an Indian rhinoceros, the Himalayan antelope, and the wild ass.
The horn itself is white at the base, black in the middle and with a sharp, red tip. It is believed to possess healing abilities. Dust filed from the horn was thought to protect against poison, and many diseases. It could even resurrect the dead. Amongst royalty and nobility in the Middle Ages, it became quite fashionable to own a drinking cup made of the horn of an unicorn, not in the least because it was supposed to detect poison.
The belief in the healing abilities of the horn is probably based on a medieval story. In this particular tale, many animals once gathered around a pool in the midst of night. The water was poisoned and they could not drink from it, until a unicorn appeared. He simply dipped his horn in the pool and the water became fresh and clean again.
Another medieval story tells of the capture of a unicorn by a maiden. The unicorn was far too fast and wild for the man that was hunting him. He could only be tamed by a maiden who sat lonely underneath a tree in the woods. Attracted by the scent of purity he would lay his head on her lap and she would rock him to sleep. Then she would cut of his horn, and leave him for the hunter and his dogs.
There have been attempts to give these tales a Christian interpretation. In the first tale the horn symbolizes the cross and the pool the sins of the world. In the second story the maiden was Maria, the unicorn Jesus Christ and the horn a representation of the unity of the Father and the Son. Jesus, embodied in the unicorn, was killed for sake of a sinful world.