Sirius let out a bitter laugh. "Yes, it's obvious now." He'd never forgive himself for missing it. "Rats-- cunning, treachery, anything associated with vermin. I don't know why we didn't see it. He was so useful as a rat, I think none of us thought about it." That didn't stop them from teasing him.

"Dogs are protective, loyal, but in the legends about ghostly black dogs they're usually omens of death." Considering how much death had been in his life, maybe that wasn't entirely wrong either.

He sighed. "Every Animagus has to register. It's carefully monitored because it was so dangerous. We were-- oh, I think thirteen when we got the idea? There was no way we'd get permission to try. So we did it on our own: me and James and-- him. If anyone found out we'd have been sent to Azkaban." Sirius's grin turned to a grimace. He'd probably already served the time he would have if they had caught him. "So only our closest friends knew. Remus knew because we did it to keep him company. Lily married James so she had to know."