Formal:
* Je suis un loup-garou. (This is the most literal translation, but it might sound a bit awkward.)
* Je suis une personne qui se transforme en loup. (This is a more natural way to say it.)
Informal:
* Je suis un loup dans la peau d'un homme. (This is a more poetic way to say it, meaning "I am a wolf in a man's skin.")
* Je suis un loup-garou, je vous jure! (This is a playful way to say it, meaning "I'm a werewolf, I swear!")
You could also use a more specific phrase depending on the situation. For example, if you're talking about a werewolf story, you could say:
* L'histoire parle d'un loup-garou. (The story talks about a werewolf.)
Ultimately, the best way to say "I am a werewolf" in French depends on the specific context and the tone you want to convey.