Animagus

An Animagus (plural: Animagi) is a wizard or witch that can transform into an animal at will. The process is notoriously complicated. If they succeed, the wizard or witch must then register themselves as an Animagus with the Ministry of Magic, although some do not to remain incognito. Illegal Animagi include the Marauders, Sirius Black, James Potter and Peter Pettigrew, who mastered the magic to accompany their friend, Remus Lupin, in his werewolf form. Animagi could sometimes find their Patronus matched their animal form. They may also find that certain characteristics of themselves would transfer on to the animal versions of themselves. For example, Professor McGonagall, a registered Animagus, became a tabby cat, with markings similar to her spectacles.