Werewolf

Werewolves were normally human in form until the rising of the full moon, at which point they would be physically and mentally transformed into a fearsome wolf-like creature that was uncontrollably violent. Werewolf bites could be deadly unless they were treated with silver and dittany at which point the wound could be sealed and the victim sentenced to life as a werewolf. Some relief could be found through Wolfsbane Potion which would allow a werewolf to retain its human mind during transformation. Lord Voldemort recruited the notoriously vicious werewolf, Fenrir Greyback, to be one of his followers.