You know what?
It would make complete sense for Regulus’ Animagus to be a cat. I mean think about it, Sirius’ is a dog. What better way to parallel that than a cat? And since Regulus is a star in the Leo Constellation and Leo is a lion and cats are notoriously linked to lions, it actually makes astronomical sense as well.
And just imagine the Marauders’ faces when they learn that Regulus went through all of the proper legal processes to become an Animagus and it turns out that he’s a cat. A cat.
He’s not even a handsome one either.
And Sirius picks on him all the time and Regulus turns bright red but secretly Sirius couldn’t be more proud of his little brother (he always is).
And I imagine that Regulus is so embarrassed by his form that he doesn’t really tell anyone. Certainly none of his big, tough, Death Eater chums who would laugh him out of the organization. Then again, maybe that would give him an excuse to leave. Because lately, he’s beginning to feel like it’s up to him to take down Voldemort.
And the time finally comes, when he’s standing on that small island in the middle of the cave, staring down at the locket. He orders Kreacher to remove it when he’s done drinking, and he tells Kreacher that if anyone in the future should interrogate him, he is to say that the Inferi dragged him under. And then Regulus turns into a cat. And he drinks. Why? I’ll tell you why. Animagi are less prone to magical effects, as evidenced by the fact that Sirius was able to resist the Dementors’ effects when he was Padfoot and the fact that James, Sirius, and Peter traveled around with a werewolf and weren’t transformed.
So he turns into a cat and he drinks.
And he stays a cat, even after Kreacher has taken the amulet and has retreated. After all, it’s best that the world thinks he’s dead. That way, they won’t come looking for him.
So Cat!Regulus goes to Sirius and he transforms long enough to explain what’s happened and Sirius agrees that he needs to stay under cover. And he does. He stays undercover at James’ house because there’s nowhere safer, what with Sirius as the Secret Keeper. James and Lily know who he is, of course, but they refer to him as ‘the cat’ in letters to Sirius, lest anyone intercept them. Regulus transforms into his human self a few times – he never could resist holding Harry – but he mostly stays as a cat, even if it means getting run over by a toddler on a broomstick.
But then, Halloween comes and disaster strikes.
By the time Cat!Regulus can pull himself out of the wreckage, he is badly hurt and has a squished face that never seems to leave. Worst off, James is dead, Lily is dead, Harry is being taken to an abusive household, and his brother is being taken to prison.
There’s nothing that he can do. There’s nobody that he can trust. There’s nobody that would trust him.
And so he goes to Diagon Alley. And he waits. And he watches. And he listens. Because he knows that Voldemort is coming back. After all, he knows about the horcruxes. And he knows that Pettigrew is out there. The man who killed James and Lily. But even worse, the man who framed his brother.
Stay with me here.
Years later, Regulus is still hanging around Diagon Alley. A few people have tried to adopt him but he intentionally sabotages each and every attempt. After all, it’s just a matter of time before news strikes. And it does, though not in the way that he expects. For all of a sudden, his brother’s face is up on the walls, in the newspapers, his name being spoken by everyone.
Of course he has. Regulus is not the least bit surprised that his brother is the first person to break out of Azkaban. He’s just surprised that it took this long. According to the news, Sirius is heading for Hogwarts. So that’s where he has to go as well. And his ticket there? A girl with frizzy hair who wants to spend the money that her parents gave her for her birthday.
And so he gets to Hogwarts and he does everything within his power to try to capture Pettigrew, the man who framed his brother. And then, his brother’s there. The reunion is a bittersweet one, filled with lots of barking, lots of purring, and lots of Dementors sweeping past. And the brothers are reunited once more. And Regulus helps Sirius break into the castle on numerous occasions while also going after Pettigrew. All ending on the night that finds them all in the Shrieking Shack, wherein Regulus sees his brother’s brother’s son pointing a wand at a surprisingly calm Sirius.
And so Regulus steps forward and plants himself on his brother’s chest, refusing to move.
Yes, my friends.
I have come at you in the past four and a half years with multiple theories as to what really happened to our little R.A.B. And this is yet another theory:
Regulus Black was Crookshanks all along.
(Which sort of goes along with my theory from last year that Crookshanks is the Potters’ cat: X Actually this makes that theory make even more sense.)