20He's An Animagus Who Looks Like The Grim
Throughout The Prisoner of Azkaban, Harry is haunted by a giant black dog wherever he goes, and the enigmatic creature is mistaken for the Grim, an omen that supposedly foreshadows the end of one’s life. As it turns out, the Grim that follows Harry around is really just Sirius in
his Animagus form.
Sirius and his buddies James and Peter had secretly become unregistered Animagi during their time at Hogwarts to help Remus Lupin whenever he transformed into a werewolf. Being emaciated and unkempt from years of imprisonment, Sirius’ Animagus form looks a lot more sinister. Plus, Harry is under a lot of stress and encounters a series of unfortunate events, so we can see how some might mistake his transformed godfather for an omen of elimination.