Anamaigus

An anamaigus is a witch or wizard that can turn into an animal.You must be registered, however some break that law.Sirius Black is an anamaigus. He is unregistered. It is different than a werewolf. A werewolf is someone that got bitten by another werewolf, and only changes during a full moon without a choice. R.J. Lupin is a werewolf. That is why the whomping willow was planted.