I feel a change. Back to a better day. The hair stands on the back of my neck, in wildness is the preservation of the world. So seek the wolf in thyself!
~ Metallica, "Of Wolf and Man", a song about becoming a werewolf.

Even a man who is pure in heart and says his prayers by night, may become a wolf when the wolfbane blooms and the autumn moon is bright
~ Jenny Williams from the Wolfman, alluding to Werewolves

Villains who change, temporary or permanently, into werebeasts such as werewolves, werecats, werehyenas, wererabbits, werebears, etc. Lycanthropes are well-known choices for villains, with the classic Werewolf being the most popular and common variant by far.