One of the biggest Harry Potter plot holes — one that bugged me for years — is how Harry (and Fred and George) never noticed Peter Pettigrew (then posing as Scabbers the rat) on the Marauder's Map. I mean, the map shows literally everything that's going on at Hogwarts, and you'd think/hope that the holder would catch important little details like that a deceased man was sleeping in Ron Weasley's bed. The story of Sirius Black and Peter Pettigrew's infamous standoff was well-known throughout the wizarding world, so it's unlikely that Fred and George wouldn't have recognized the name on the map and questioned it if they'd seen it. So why, when Ron arrived at Hogwarts with "Scabbers," and for the years following, did no one notice that someone who was supposed to be dead was actually roaming the corridors of the castle?
A theory on Tumblr makes complete sense, considering that the creators of the Marauder's Map were illegal Animagi. James Potter, Sirius Black, and Peter Pettigrew wouldn't have wanted to show up on the map while on their numerous and highly dangerous adventures with Remus Lupin, so it's likely they used their remarkable magical skills to protect their identities. In doing so, they also blocked other Animagi from showing up on the map — but only when they were in animal form.
Since Pettigrew was obviously forced to stay in rat form throughout the duration of his time at Hogwarts with Ron, he wouldn't have showed up on the map when Fred, George, and later Harry, were using it. This also explains why Rita Skeeter was able to flit around the castle in beetle form digging up dirt on everyone she could without being found out on the Marauder's Map. Read the full theory and explanation ahead.