here, and we also recently finished a David Attenborough documentary on wolf packs. Which makes a werewolf pretty irresistible. So. Werewolves!
Shapeshifters appear in many cultures, including Native American, but usually involve a lot of animals (ravens, deer, etc). Werewolves, however, come mostly from European folklore. Wolves could get scary in the winters, especially when people encroached on their forest territory and built increasing numbers of farms with tempting tasty livestock. So wolves became symbols of evil, and it was really scary to think about a wolf with extra speed and strength and human intelligence. Werewolves traditionally transform with the moon, and are usually made because of the bite or scratch of another lycanthrope.
I'd write more, but I have to go teach, where I might (actually) use the game of werewolves to teach analysis. Whee!