How did the werewolve myths originate?

The concept is probably prehistoric. The trope of people turning into animals is found in the folklore of most cultures around the world; is a core concept of shamanism and clan structure. (The Hurons clans were based on animal ancestors).

The earliest instance of a man turning into a wolf is from the Kingdom of Arcadia. (An aboriginal culture of Greece that predates the Greeks). King Lycaon offered human sacrifice to Zeus who when he discerned it to be human flesh turned him into a wolf. It is from Lycaon that we have the term Lycanthropy.