Scabbers

Scabbers, formery known as Mister Peter Pettigrew, Senior Marauder, Order of Merlin, First Class, , is a wizard and animagus of unknown whereabouts as of Summer 1993.

Background
Scabbers's past up until 1991 is assumed to be the same as his canon counterpart; a very competent but cowardly wizard, he used to be one of the original Marauders but betrayed the Order of the Phoenix under duress from the Dark Lord, and hid out in the form of a rat for over a decade, with his friend Sirius Black blamed for his crimes instead.

1991-1992
In 1991, Scabbers had become an unaware Ron Weasley's pet, and was taken aboard the Hogwarts Express by Ron. Ron then attempted a possibly phony Color-Changing Charm created by the Twins on him, meant to turn him yellow; it failed.

Early on in the year, Scabbers saw Ron attempting to prepare a prank on the Twins in retaliation for another. How much of a wrong way Ron was going about it ruffled the old Marauder who, out of a combination of loneliness and his itch to correct Ron's mistakes, revealed his human form and began giving him advice. He claimed to be a rat who could turn into a man, rather than the opposite, and did not give any information about his past as Peter Pettigrew; Ron was initially skeptical but was increasingly nonplussed by the oddity of the situation the more time he spent with Hermione Granger.

Throughout the year, Scabbers helped the Weasley boy design all sorts of practical jokes to finally give payback to the Twins for all that they had done through the years. He handled most of them himself, but he did "teach Ron a thing or two along the way", and an odd sort of friendship grew between the two. However, when Ron and Hermione (along with Harry Potter) began seriously discussing the possibility of Lord Voldemort being in the Castle, all of his old cowardice came back to him; he grew more and more restless, torn between helping his young friend and saving his own skin, and again he made the cowardly choice, disappearing overnight at the end of the year with a succinct, apolegetic farewell note. (Only late in the next year would Ron finally reveal the truth about Scabbers, following the escape of Sirius Black from Azkaban.)