Okay, before you read this, I have to warn you of three possibilities. (If you want to get straight to the point, skip the numbered points)

1]  Maybe this entire post is a temporary side effect of spending the past two hours discussing far-fetched conspiracy theories with my father and brother. I'm questioning everything right now.

2] Maybe this is a crazy 2 AM thought that I will regret posting in the morning. If that is the case: Dear future-self, you don't have to be embarrassed of all this because you have like two readers, yeh irrelevant shit.

Or (which seems super likely right now)

3]  I COULD BE ON TO SOMETHING.

So first of all, it's always mentioned Crookshanks seems suspiciously smart. Too smart for a cat. He figured out Scabbers' true identity, helped Sirius into Gryffindor Tower, etc, etc.
I don't think a cat could ever do that, no matter how magical it was or whatever.

In Goblet of Fire, He looks disapprovingly at Harry and Ron when they're cheating on their Divination homework. HOW?? This feels like enough proof. Cats can't read!!  How the frickity frackity else would he know they were cheating?

Then, in Deathly Hallows, there's no canonical evidence suggesting where Crookshanks went while the Golden Trio were on the horcrux hunt. Harry Potter wiki says most people assume Ginny took care of him and he stayed at the Burrow while some think he went off alone. Where did he go? To fight off against Voldemort in some way, duhh.

I'm honestly surprised how nobody finds this suspicious. Maybe Sirius or Hermione did know, but then why didn't they tell the others? I know this contradicts everything I said above but I don't know, there could be a plausible explanation .

Now here's a theory I have as to why Crookshanks had to stay undercover or why Sirius and Hermione didn't disclose his actual identity.

Crookshanks went to school with the marauders. He was a good friend of the marauders' but turned bad later on. He knew they were animagi or found out later on.

He joined the Death Eaters and did terrible things. Though as soon as he heard about Voldy's plans to kill one of his ex-friend's baby, he started working against that. Not the movement entirely, (he still wanted to be a Death Eater) but  he did want to prevent the  death of an innocent child  because he did have some  good left in him. It was him that suggested Sirius to make Pettigrew the Potters' secretkeeper, maybe anonymously or through someone else. Of course, he wouldn't know Peter was a Death Eater. Not all Death Eaters are aware of each other. 

Later, he found out what happened. Peter's betrayal, Sirius sent to azkaban, Voldemort's downfall, the whole thing.

They were rounding up Death Eaters and he was a quite famous and notorious one so he took a leaf out of Peter's book. Staying under the guise of an innocent stray cat in Diagon Alley probably made him come across all sorts of information, considering how many wizards go there. It was a really strategic point. He didn't even let anyone purchase him until the Golden Trio visited and he saw Scabbers ( Peter).

He immediately started trying to catch him because he heard the whole story, he recognized the missing toe. Remember, even Lupin was skeptical about Scabbers's identity until Sirius pointed the toe out?

Anyways, he helped Sirius get into Gryffindor tower because he did want to see Peter pay for his betrayal.

Hermione and Sirius dismissed his behaviour under the assumption that it was due to his magical properties or something.

Spending all the time with the Golden Trio changed him a lot and he decided to be on the good side when Voldemort rose again.

He couldn't reveal his identity to the others because they wouldn't accept him. He had done way too much horrendous stuff in his Death Eater days. Therefore, Crookshanks worked against Voldy and friends behind the scenes.  Maybe he died while doing it which is why we hear no more of him.

That is all!!

What do you think?

xoxo
Nushie