What is a Black Unicorn?

A black unicorn is almost identical to the traditional white unicorn, with the key difference that it is, of course, entirely black. Much like a black horse is to a white horse, the black unicorn has a different color of coat. Black unicorns, however, are believed to be quite rare compared to white unicorns and are not often mentioned in fantasy novels. The horn of the black unicorn is also black.

The second novel in the Magic Kingdom of Landover series by Terry Brooks is titled "The Black Unicorn" and concerns such a mythical beast.