We know that the cat 1) lived very long, 2) hated the rat, probably because he knew it was Pettigrew, 3) understood, when they spoke about it. So I guess it's an animagus.
R.A.B. is probably Sirius' brother. Having an animagus brother makes it more likely that he himself was (is?) an animagus. Besides, nobody knows what happened to him. All we know is that he disappeared after turned against Voldemort.
Discussion (3)
Hmmm, interesting. It's been a while since I read the books, so my memory is fuzzy. But I do recall expecting Crookshanks to turn out to be an animagus when Scabbers did. So, I wouldn't be at all surprised if he turned out to be one later. Also, Fawkes is a likely candidate for animagusiness.
About Fawkes... Even if it sounds crazy, it occured to me more than once that he's a corcrux of Dumbledore...
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Fawkes is a corcrux of DumbledoreI don't think so. It would be either obvious or boring. Crookshanks may be something more than a mere household cat, but another animagus as a pet of a friend of Harry would be ... lame.