Ron Weasley absolutely hated Hermione's squash-faced cat Crookshanks, but that spunky kitty could see right through people — and other animals — from the get-go.
Remember how horribly Crookshanks would treat Scabbers, Ron's pet rat? Scabbers ended up being the traitorous Peter Pettigrew who was the reason for Harry's parents' deaths and the return of the malicious Lord Voldemort.
This fan theory suggests that Crookshanks might have previously belonged to Lily Potter. There isn't much concrete evidence to truly support this theory, but a few tidbits from the book do, surprisingly, add up.
For instance, in Harry Potter And The Deathly Hallows, Harry and the gang are hanging in Grimmauld Place when Harry finds a letter that Lily had sent Sirius a few months before she was murdered. The letter reads:
Thank you, thank you, for Harry's birthday present! It was his favourite by far. One year old and already zooming along on a toy broomstick, he looked so pleased with himself, I'm enclosing a picture so you can see. You know it only rises about two feet off the ground, but he nearly killed the cat and he smashed a horrible vase Petunia sent me for Christmas (no complaints there). Of course, James thought it was funny, says he's going to be a great Quidditch player, but we've had to pack away all the ornaments and make sure we don't take our eyes off him when he gets going.
This note mentioned a cat, and while there is nothing distinguishable that might point to it being Crookshanks, these details might:
1. Crookshanks Truly Hated Peter Pettigrew
Crookshanks immediately knew who the seemingly innocent rat Scabbers was the second he met him. Perhaps Crookshanks wanted to kill Peter Pettigrew because of who his previous owners were, not only because he was trying to protect Hermione and her friends.
2. Crookshanks Is A Half-Kneazle
You might be wondering how Crookshanks would have been alive back when Harry's parents were still alive in 1981 if he was Hermione's pet. Crookshanks is half-Kneazle, which is a magical, cat-like creature that tends to interbreed with cats. The Kneazle part of Crookshanks makes him clever, protective and intelligent enough to communicate with Sirius. The Kneazle blood would also give the cat more than just nine lives.
3. Crookshanks Had Been At The Pet Shop For A Long Time
Hermione explains to Ron that Crookshanks had been at the pet shop for "quite some time" because "nobody wanted him." This would be because Crookshanks was biding his time for those 12 years until he could actually get revenge.
4. Crookshanks Seemed To Immediately Help Sirius
Many people think that if Crookshanks had been previously owned by the Potters, Sirius would have said something. Perhaps he did notice, but didn't mention it. Sirius might not have wanted to burden Harry with that fact, as any talk of his parents might hurt him.
Also, Crookshanks seemed so quick to help a random dog out, so that must mean they must have had some prior interaction with each other.
J.K. Rowling has yet to deny or confirm this theory, but regardless, it's a fun one to mull over. Rowling has a way of making many bits and pieces connect in the Harry Potter books, and this could possibility be another one.
Even if it isn't true at all, Crookshanks deserves to be pet and scratched all day long for being a good, little boy.