"But Professor Lupin… Scabbers can’t be Pettigrew… it just can’t be true, you know it can’t…” “Why can’t it be true?” Lupin said calmly, as though they were in class, and Hermione had simply spotted a problem in an experiment with grindylows. “Because… because people would know if Peter Pettigrew had been an Animagus. We did Animagi in class with Professor McGonagall. And I looked them up when I did my homework—the Ministry of Magic keeps tabs on witches and wizards who can become animals; there’s a register showing what animal they become, and their markings and things… and I went and looked Professor McGonagall up on the register, and there have been only seven Animagi this century, and Pettigrew’s name wasn’t on the list—”

Hermione just found out that Sirius was an animagus but wasn't on the list. And yet, her objection was that Peter wasn't on the list so he couldn't be an animagus.