From Bulfinch's Mythology:
THE UNICORN
Pliny, the Roman naturalist, out of whose account of the unicorn
most of the modern unicorns have been described and figured,
records it as "a very ferocious beast, similar in the rest of its
body to a horse, with the head of a deer, the feet of an
elephant, the tail of a boar, a deep bellowing voice, and a
single black horn, two cubits in length, standing out in the
middle of its forehead." He adds that "it cannot be taken
alive;" and some such excuse may have been necessary in those
days for not producing the living animal upon the arena of the
amphitheatre.
The unicorn seems to have been a sad puzzle to the hunters, who
hardly knew how to come at so valuable a piece of game. Some
described the horn as moveable at the will of the animal, a kind
of small sword in short, with which ho hunter who was not
exceedingly cunning in fence could have a chance. Others
maintained that all the animal's strength lay in its horn, and
that when hard pressed in pursuit, it would throw itself from the
pinnacle of the highest rocks horn foremost, so as to pitch upon
it, and then quietly march off not a whit the worse for its fall.
But it seems they found out how to circumvent the poor unicorn at
last. They discovered that it was a great lover of purity and
innocence, so they took the field with a young VIRGIN, who was
placed in the unsuspecting admirer's way. When the unicorn spied her, he approached with all reverence, couched beside her, and laying his head in her lap, fell asleep. The treacherous virgin
then gave a signal, and the hunters made in and captured the
simple beast.
Modern zoologists, disgusted as they well may be with such fables as these, disbelieve generally the existence of the unicorn. Yet there are animals bearing on their heads a bony protuberance more or less like a horn, which may have given rise to the story. The rhinoceros horn, as it is called, is such a protuberance, though it does not exceed a few inches in height, and is far from agreeing with the descriptions of the horn of the unicorn. The nearest approach to a horn in the middle of the forehead is exhibited in the bony protuberance on the forehead of the giraffe; but this also is short and blunt, and is not the only
horn of the animal, but a third horn standing in front of the two
others. In fine, though it would be presumptuous to deny the
existence of a one-horned quadruped other than the rhinoceros, it may be safely stated that the insertion of a long and solid horn
in the living forehead of a horse-like or deer-like animal, is as
near an impossibility as any thing can be.