"That's not a rat," croaked Sirius Black suddenly. (PA17)
Rats are medium-sized, long-tailed rodents, sometimes kept as pets, but often regarded as vermin or pests.
Peter Pettigrew lived as a rat for twelve years, as a pet of the Weasley family known as Scabbers.
As a rat, Scabbers once bit Goyle's finger on the Hogwarts Express (PS6).
Norbert ate dead rats by the crate-load when he was a growing dragon kept in Hagrid's hut (PS14).
In the Magical Menagerie was a cage full of black rats which were busy jumping rope with their tails; the saleswitch implied that they would live longer than a common rat's three-year lifespan (PA4).
During detention in Snape's dungeon, Harry and Ron spend two hours pickling rats brains (GF19).
When Sirius was hiding in a cave near Hogsmeade during the Triwizard Tournament, he stayed mostly in his animagus form and was living off rats (GF27).
While living in hiding in number twelve, Grimmauld Place, Sirius appears to have kept Buckbeak mainly on a diet of dead rats (OP6).
In Harry's OWLS he was supposed to turn a rat orange but mixed up the incantations and caused it to swell to the size of a badger instead (OP31).
Several different rat parts are used as potion ingredients, including rat tails and rat spleens.
According to the letter Harry receives in Philosopher's Stone, rats are not on the list of approved list of pets first years are permitted to bring to school, yet students seem to bring them in spite- or perhaps, in defiance of, the rules.