Four Unregistered Animagus

'There were witnesses who saw Pettigrew die,' he said. 'A whole street full of them...'
'They didn't see what they thought they saw!' said Black savagely, still watching Scabbers struggling in Ron's hands.
'Everyone thought Sirius killed Peter,' said Lupin, nodding. 'I believed it myself - until I saw the map tonight. Because the Marauder's Map never lies... Peter's alive. Ron's holding him, Harry.'
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Then Hermione spoke, in a trembling, would-be calm sort of voice, as though trying to will Professor Lupin to talk sensible.
'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 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 only been seven Animagi this century, and Pettigrew's name wasn't on the list -'
Harry barely had time to marvel inwardly at the effort Hermione put into her homework, when Lupin started to laugh.
'Right again, Hermione!' he said. 'But the Ministry never knew that there used to be three unregistered Animagi running around Hogwarts.'

MOONY, WORMTAIL, PADFOOT AND PRONGS - Pg 373