Stories We Like: A Guide to the Comma
mid-14c., "intimate, very friendly, on a family footing," from Old French famelier, from Latin familiaris "domestic, of a household;" also "familiar, intimate, friendly," dissimilated from *familialis, from familia (see family). The sense gradually broadened. Of things, from late 15c. The noun meaning "demon, evil spirit that answers one's call" is from 1580s.
in Western demonology, small animal or imp kept as a witch's attendant, given to her by the devil or inherited from another witch. The familiar was a low-ranking demon that assumed any animal shape, such as a toad, dog, insect, or black cat. Sometimes the familiar was described as a grotesque creature of fantasy, an amalgam of several creatures.