The animal often depicted as a witch’s familiar and popular Halloween costume is a black cat.
In European folklore, a familiar was a supernatural helper believed to assist a witch or cunning person with magic, divination, protection, or spiritual work. Historical accounts described familiars in many forms, but small animals—including cats, dogs, rats, birds, frogs, and hares—were especially common. The “black” part belongs mainly to later cultural imagery rather than to a universal rule about familiars.
Cats became strongly associated with witchcraft in medieval and early modern European storytelling. Their nocturnal habits, silent movement, reflective eyes, and independence made them seem mysterious to observers. During witch trials, ordinary animals could be reinterpreted as evidence of supernatural partnerships, although the beliefs varied by place and period.
Modern Halloween imagery simplified this complicated folklore into a recognizable character: the witch with a pointed hat, broomstick, and black cat. An owl, bat, or spider can also appear in Halloween art, but the black cat is the conventional familiar and the fairest single answer.