A unicorn is a mythological equine with a single, usually spiraled horn growing from the center of its forehead. Classical unicorns were technically not equines, having thin, almost deer-like bodies with cloven hooves and a lion-like, tufted tail. The modern convention is to depict them as a standard horse with a horn on the forehead.
Some sources argue that the apparently single horn (known as an alicorn) is really a pair of horns growing spiraled tightly about one another. Legend has it that a unicorn's horn can purify water and detect poison, thus making cups allegedly made from unicorn horns highly prized objects. However, in reality, these cups are carved from other sources, such as narwhal teeth.
Since the Middle Ages, unicorns traditionally are said to be attracted to virgins and the morally pure, and were sometimes used as a symbol for Christ in medieval paintings.
The karkadan is a highly aggressive unicorn from Asia and Arabia possibly derived from a now extinct rhinoceros. The kirin is also often portrayed as a variety of unicorn in modern times, but the original versions possessed the standard two horns. Other types of unicorn-like creatures also exist in folklore and legend, from the miraj to the Invisible Pink Unicorn.
For the purposes of e621, unicorns are tagged as equine but should not be tagged as either horse or pony. If a creature is an equine and possesses both a horn and wings, it should be tagged as winged unicorn instead.