An Imp is a small demon that a witch keeps in a bottle. Imps are used for magical purposes like carrying out tasks and spells. Imps could shape-shift into rodents, toads or frogs, and insects like flies and spiders. In return for their work, they suck the blood from the witch.
Witches were checked for Witch Marks, this was a strange lump, warts, discolored skin, from which the witch would feed her Imp.
When nothing was found, it was agreed that she used her finger.
Prison cells were filled with rats and insects during the witch trials finding any animals in a suspected witch’s cell was suspicious. Guards tried to kill the animal, if it was killed, it was a normal animal, if it escaped that meant it was the witch’s Imp.
Imp and familiar were used interchangeably.