Harry Potter and the Sorcerer's Stone
Fluffy is a large, vicious, three-headed dog who was once cared for by Rubeus Hagrid. Fluffy's greatest weakness is the inability to resist falling asleep to the sound of music.
Rubeus Hagrid originally purchased Fluffy from a "Greek chappie" at a pub. Hagrid enlisted Fluffy's aid in guarding the Philosopher's Stone during the 1991–1992 school year.
Fluffy was generally vicious, quite fierce, and extremely strong, though he possibly acted somewhat more docile around certain people, like Rubeus Hagrid, who was able to get Fluffy into the third floor corridor. Three-headed dogs are very rare creatures, and it is unknown if, like the Runespoor, each head serves a different purpose.