On the Marauder’s Map, Peter Pettigrew
Despite how grim it becomes towards the end, Harry Potter is ultimately a children’s series. It’s designed for young children. This implies that not everything has been carefully considered. There are a few story holes, and J.K. Rowling probably never imagined her novel would become so popular that adults would pore over it and pick out every inconsistency.
But, hey, they did it, and now we’re here. One big one is the possibility that Peter Pettigrew was found on the Marauder’s Map. This would have established him as Ron’s pet rat without a doubt.