The fan theory: The Marauder’s Map plot hole isn’t a plot hole after all
Every Potter fan worth their salt has pointed out this particular inconsistency: if Scabbers was really Peter Pettigrew the whole time, why didn’t Fred and George notice him on the Marauder’s Map? Sure, the twins were pranksters, but there’s “bewitching someone’s prefect badge” and then there’s “letting your brother keep a mass murderer as a pet for three years, SIKE.”
Before Pottermore, J.K. Rowling explained on her website that the twins used the map solely for purposes of magical mischief. They weren’t too fussed about who was or wasn’t hanging around the castle. Besides, she said, Hogwarts is a big school with many students and faculty members, and therefore many moving dots. It’s not outrageous to assume Fred and George would miss one.
I always found this explanation lacking, to be honest, and I’m not alone. That’s why this fan came up with an alternate theory, one we’ve discussed before: if someone is in their Animagus form, they only show up on the Marauder’s Map if the person perusing the map already knows they are an Animagus. Lupin was the only one who saw Pettigrew on the map, after all, and no one ever noticed Rita Skeeter. It makes sense. At the very least, it makes more sense than “Well, Fred and George just weren’t very detail-oriented.”