I'm trying to find a story and I can't remember the name or the plot really but Harry Potter has a large cat, like a Serval or a Savannah Cat or some other large cat of that magnitude that travels around school with him. I think the cat also rides around on his shoulders. Mrs Norris is afraid of this cat, and in a later chapter Fred and George end up asking to borrow the cat to set up a prank without having Mrs Norris come near them. While they do that Harry ends up playing his own prank on Mrs Norris while they do this. The prank resulted in Mrs Norris making fart sounds anytime she moved or walked. I think it’s a Harry Potter AU where something was changed before his first year of only because Harry has a cat at Hogwarts.
I think I just kind of looked at the last updated chapter of a fanfic, so I can’t tell you what else was going on in it, but I’d like to know what fic it is so I can read it all. My best guess is that it was on FanFiction.Net and had at least 40k+ words. The characters listed for it included Harry Potter.