The three-headed dog guarding the trapdoor at Hogwarts is named Fluffy.
Fluffy appears in Harry Potter and the Philosopher’s Stone as part of the protections placed around the Philosopher’s Stone. Hagrid obtains the dog from a Greek man in a pub and later keeps it near the trapdoor on the third-floor corridor. The dog’s three heads make it an intimidating guard, but music can lull it to sleep.
Harry, Ron, and Hermione discover that Fluffy is guarding a trapdoor after following clues about the restricted corridor. When they return to confront the protections around the Stone, Hermione plays music to put Fluffy to sleep before they descend.
Fang is Hagrid’s boarhound, Norbert is the dragon Hagrid hatches, and Aragog is the giant spider connected with the Chamber of Secrets. All are animals in the books, but only Fluffy guards the trapdoor.