Gift of Vengeance
Betty, disguised as a pale green magical snake, has helped Harry to survive until he comes to Hogwarts. Sirius is believed to be guilty of blowing Pettigrew to pieces, and when she finds out that Pettigrew costed her best friend his entire family, she seeks vengeance. She has found the rat before Christmas first year, and Harry is invited to spend Christmas with Neville, so she as a snake bites the rat, puts the human corpse in a rarely used hallway with the shoes barely peaking around a corner, cuts into his forehead 'rache' in runes (as an Easter egg for the Sherlock Holmes story where the killer wrote it on the wall when he took his revenge), with his own blood writes on the wall
'Padfoot
I gift you vengeance for Prongs and Lilyflower
Merry Christmas
F&G'
with a spell that won't come off, takes a few photos to anonymously send to the Prophet with a chilling rhyme and puts the whole hallway under a time-released preservation and notice-me-not charms that dissolve two days after Harry leaves for Longbottom Manor.
Understandably, the Wizarding world doesn't take well the fact that a hero that was supposed to be dead for a decade suddenly turns up as a fresh corpse with a completely healed stump instead of a finger and obvious signs of revenge murder. At first Fred and George are blamed, and thanked, for the murder, until Harry accidentally lets slip to his new guardian Sirius that he had heard of the Marauders and wanted cool codenames for himself and his snake, so they sign their mischief with the initials of those codenames - Fawn and Greenery.