In Harry Potter and the Prisoner of Azkaban, Hermione buys a cat named Crookshanks. You might remember him. Throughout the book, Crookshanks tries to attack Ron's rat, Scabbers. But other than this book, he does not have a major appearance. He had a VERY small appearance in the Deathly Hallows, but he might have been mentioned at one point, but you didn't even notice. This is going to be another mind-bending theory for you. But it's short.
In Deathly Hallows, Harry, Ron, and Hermione are staying at Privet Drive and Harry stumbles upon a letter Lily had sent to Sirius. Let's take a look at one sentence involving Harry's toy broomstick: "You know it rises only two feet off the ground, but he nearly killed the cat and he smashed a horrible vase Petunia sent me (no complaints there)." This shows that Lily and James had a cat when Harry was just an infant.
Voldemort came and he murdered Lily and James. The house was destroyed, but what could have happened to the cat? My theory is that it wandered off to Diagon Alley or was picked up by a witch who believed he was a stray. And the cat would have ended up at the Magical Menagerie and he might have been there when Hermione walked in to buy a cat.
At one point, Hermione says after buying him, "Poor Crookshanks, that witch said he’d been in there for ages; no one wanted him." That's because he was there since the early '80's.
Lily and James were friends with Sirius and Pettigrew. Lily and James had a cat and in the third book, Crookshanks is shown to have a prior relationship with both Sirius and Pettigrew (in their Animagus form). And one more thing: Crookshanks could not have been in Godric's Hollow when Voldemort killed Lily and James, because he might have become a Horcrux. So when he saw Voldemort coming, he could have escaped (or something) and went to Sirius to mime what happened (animals can mime, and Crookshanks is supposedly very intelligent).
And look at what Hagrid said in the third book: "Jus’ got him outta the ruins, poor little thing, with a great slash across his forehead, an’ his parents dead… an’ Sirius Black turns up, on that flyin’ motorbike he used ter ride. Never occurred ter me what he was doin’ there. I didn’ know he’d bin Lily an’ James’s Secret-Keeper… white an’ shakin’, he was."
Sirius was there because he knew what happened, and he wasn't their Secret-Keeper.
And that's the theory! Do you like it? Agree? Anyway, follow me on Twitter. I'm @JonathanESim, and comment your thoughts below.