Fluffy is one of Hagrids many pets. He was stationed in a room in Hogwarts castle to protect the trapdoor that lead to the chamber to get the Sorcerer's Stone. Fluffy took a bite out of Snape (Hurray for Fluffy!) and did a wonderful job protecting the door. Eventually, Hermione put him to sleep (by playing Harry's flute) and Harry was able to reach the stone.