A wicked witch is one of the most evil kind of witches. Several wicked witches lived in the Land of Oz. Lolotte and Dahlia were wicked witchs as well.
There is a belief, not so common anymore, that a wicked witch can can throw a magic bridle over someone's head and turn the person into a horse. The witch then rides the unhappy person a long distance to some witches' gathering, and rides back in the morning. The victim soon becomes ill and weary from lack of sleep and died. A number of folktales exist that tell how some victims were able to escape this fate.
A male equivalent may be a wicked wizard or wicked enchanter.