"I thought you'd come and help your friend," he said hoarsely.
"An Animagus," said Black, "by the name of Peter Pettigrew
"That's not a rat," said Lupin quietly. "He's a wizard."
"The screams and howls the villagers used to hear were made by me."
After the dog drags the trio into the Shrieking Shack it is revealed the dog is sirius black.
Lupin insists that he only learned the truth about Sirius tonight, when he saw someone who shouldn't be alive on the Marauder's Map.Sirius and Lupin then drop the bombshell: Ron's rat Scabbers is really Peter Pettigrew.
"So that's why Snape doesn't like you," said Harry slowly, "because he thought you were in on the joke?"
Lupin tells everyone to gather round for story time and starts to explain everything.Peter, along with Sirius and James, all became unregistered Animagi while they were attending Hogwarts.
"That's right," Severus Snape was pulling off the Invisibility cloak
The gang started wandering around in their animal forms and ended up writing the Marauder's Map based on their explorations of Hogwarts.
"My best friends became animagi to accompany me."
Lupin explains to the kids that Sirius played a joke on Snape one time and lured him to the Shrieking Shack while Lupin was in his werewolf form. James pulled Snape back at the last second, or else Lupin would have bitten Snape.
Dumbledore forbade Snape from telling anyone the truth about Lupin. We can see why the guy held a grudge.