A Harry Potter fan theory has suggested that Hermione's cat once belonged to the Potter family. Hermione took ownership of Crookshanks before beginning her third year at Hogwarts. Crookshanks was a highly intelligent cat that proved to be useful in exposing a key enemy.
Crookshanks was a ginger feline with a lion-like appearance and bright yellow eyes. He was half-Kneazle, which meant he had the ability to detect untrustworthy individuals. The magical pet was also highly independent which made him the perfect pet to take along to Hogwarts. Hermione originally went to Magical Menagerie to pick out an owl. The saleswoman that shared that Crookshanks had been in the shop for a very long time. Hermione couldn't pass up the chance to adopt Crookshanks.
The fan theory posited the notion that Crookshanks ended up at Magical Menagerie because his original owners were no longer around to care for him. In Harry Potter and the Deathly Hallows, Harry read a letter written by his mother, Lily. In it, she mentioned a cat, making Harry wonder if it survived the attack by Lord Voldemort. It's assumed that after Voldemort killed James and Lily, the cat ran away and eventually wound up at Magical Menagerie. That was until Hermione came along.
More evidence of the theory was provided by Crookshanks' interactions with Scabbers, Ron's pet rat. Scabbers was technically Peter Pettigrew in his Animagus form but that wasn't known at the time. Upon meeting Scabbers, Crookshanks detected he was a fraud. Crookshanks continuously attacked or chased Scabbers throughout the Gryffindor dormitories. The cat eventually encountered Sirius Black in his Animagus form and tried to help lead him to Scabbers. Sirius was on the search for Peter for his involvement in the death of Harry's parents.
Crookshanks took a liking to Harry and would often sit on his lap while lounging in the dormitory. This could have been because the cat remembered Harry from when he was a baby. Although Kneazles were known for detecting untrustworthy people, Crookshanks seemed extra determined to expose Peter as Scabbers. If he was the Potter family's cat, he would have witnessed James and Lily's murder. He might have been able to sense Peter's involvement which was why the cat didn't give up his hunt.
J.K. Rowling has never commented on the theory regarding Cookshanks, but that doesn't mean it was out of the realm of possibility. The same goes for other theories such as the belief that Hagrid was Voldemort's secret Death Eater. The author has been known to reveal game-changing tidbits in the years since the series ended. Rowling also has a habit of changing Harry Potter canon.
