Ron Weasley’s pet rat, until he wasn’t anymore.
Scabbers was actually Peter Pettigrew in his Animagus form.
After tricking Sirius, Pettigrew transformed into a rat and stayed that way for twelve years, from November 1981 to June 1994, living as Percy and then Ron’s pet, Scabbers, in order to hide from the Death Eaters who thought he had betrayed Voldemort.
As Scabbers, Pettigrew was not fond of Hermione's cat, Crookshanks, and with good reason. Crookshanks is part Kneazle, which means that he can sense duplicity. He saw right through Peter's disguise and tried to attack him whenever he got a chance.
'scabby' means either covered in scabs or loathsome and despicable. Seems an odd name to give a pet.