Another Animagi from the Harry Potter series, Sirius chose to take form as a 'shaggy, black dog'. Not only was he able to play with his close friend Moony, but he was also large enough to keep him in check when his friend's Werewolf side kicked in gear. A true friend Sirius Black is, instead of running in fear at the truth of his friend Moony, he learned how to illegally change himself into a less edibly desirable creature than a human--hence the shaggy black dog. PS I gave him some defining features(ie. White patch over eye and random white freckling on his toes, chest and rear thigh), where-as Rowling left out much of the description of Sirius' Animagi form, usually only reffering to him as a 'shaggy black dog'.