The popular conception of Werewolves is derived from the much larger concept of shape-shifters. Shape-shifters include any human, witches, sorcerers, or other magically empowered people who can transform into another creature. Were is the Old English word for man, thus the prefix were- is applied to a shape-shifter in animal form (were-wolf, were-bat, were-rabbit, etc.)
Sometimes this transformation came in the form of a curse as when the sorceress Circe turned Ulysses’s men into swine in The Odyssey. In another classical tale Golden Ass, the protagonist uses a magic ointment which turns him into an ass. The act of shifting may be voluntary through the use of ointments or magic, or involuntary as the typical Werewolf story.
Shape-shifting is a prominent concept in many cultures especially Norse, Scandinavian, and Teutonic.