Sirius Black’s Animagus transformation takes the form of a black dog in the Harry Potter books.
Sirius secretly learned to become an Animagus while he was a student at Hogwarts. His friends James Potter and Peter Pettigrew also became Animagi, allowing the group to accompany Remus Lupin during his werewolf transformations. Sirius’s animal form resembles a large black dog and helps him escape from Azkaban.
Before Harry learns Sirius’s identity, the dog is associated with a frightening omen sometimes called the Grim. Sirius later uses his Animagus form to travel and hide, including during events connected with the Shrieking Shack.
Sirius’s form is different from James Potter’s stag form, which gives James the nickname Prongs. Peter Pettigrew becomes a rat, while Remus Lupin is not an Animagus; he transforms into a werewolf because of lycanthropy.