What animal can Sirius Black transform into?

The story behind the answer

Dog is correct because Sirius Black is an unregistered Animagus who can transform into a large black dog, earning him the nickname “Padfoot.” He learned the skill at Hogwarts with James Potter (Prongs) and Peter Pettigrew (Wormtail) to support their friend Remus Lupin during his werewolf transformations—hence, not a wolf himself. Sirius also used his dog form to evade detection and ultimately escape Azkaban.

To remember this, think: Sirius is the “Dog Star” in the constellation Canis Major—his very name points to his animal form. Plus, “Padfoot” is a folkloric term for a ghostly black dog, matching his disguise that people mistook for a Grim, an omen of death.