In Harry Potter and the Prisoner of Azkaban, Sirius Black transforms into a black dog. His animal form reflects the nickname Padfoot and helps explain sightings of a large dog near Harry and Hogwarts.
Sirius is an Animagus, a wizard who can transform into an animal at will. He learned the difficult skill while at Hogwarts so he could accompany Remus Lupin, whose werewolf transformations required protection and companionship.
The black dog is not the same kind of transformation as a werewolf’s involuntary change. Sirius’s form is controlled, and the film eventually reveals that he is Harry’s godfather rather than the murderer many characters believe him to be.