Scabbers is the
odd name for
Ron Weasley's pet rat in the
Harry Potter series by
J.K. Rowling. Normally, a pet rat would be used as a
vehicle for fleshing out a character, but Scabbers actually was a major
plot point in
Harry Potter and the Prisoner of Azkaban.
Scabbers was actually a semi-evil wizard who was an animagus, or a person who could morph into a specific animal. His real name was Peter Pettigrew, a dull lump of a man who was originally friends of Harry Potter's father, James, and his crew of misfits, which included Sirius Black and Remus Lupin.
After Lily and James Potter were killed by Voldemort, who was tipped off as to their whereabouts by Pettigrew, Sirius Black went gunning for Peter. When Sirius caught up with him, Pettigrew chopped off his own finger and created a large explosion, leaving everyone to assume Sirius had killed him. Black was sent to Azkaban Prison for the murder of Pettigrew, and for collaborating with Voldemort, which was another false charge.
Pettigrew could no longer wander around in human form, so he changed into a rat. Somehow, he was picked up by Percy Weasley, who eventually gave Scabbers to Ron Weasley, his brother. Scabbers just hung out for years, until by chance Harry Potter and Ron became friends.
When Sirius Black broke out of prison in book (and movie) three, he found out where Peter was, and tracked him down. Everyone assumed he was trying to get at Harry, but it was Scabbers he was after. In the end, Scabbers was found out, he turned back into Peter Pettigrew, and he became the instrument through which Voldemort was resurrected.