There's 'Harry's Cat Transformation'. This fanfic might explore how Harry having cat ears and a tail changes his daily life at Hogwarts. Maybe he has enhanced senses now, which could be both a blessing and a curse in the magical school environment. For example, he could hear things he doesn't want to, like secrets in the castle.
The plot could involve a dark magic spell gone wrong. Harry gets cursed with cat ears and a tail. He then has to find a way to reverse it while dealing with the strange new sensations and the reactions of his friends. Some might find it hilarious, while others might be worried about him.
One popular 'Harry has dog ears' fanfiction might be on platforms like Archive of Our Own. You could search using those keywords to find stories where the author has creatively given Harry dog ears and explores the implications in the Harry Potter universe.
Perhaps it's a story where Harry is born with cat ears due to some strange magic in his family's past. He could be an outcast because of it at first, but then he discovers that his cat ears are actually a sign of a special power. This power could be related to communicating with magical creatures, especially cats, which then helps him in his adventures. He might use this newfound ability to gather information or even form an army of magical cats to fight against the dark forces.
Well, Kakashi might tease Naruto a bit about his new look. He could say something like 'Naruto, those cat ears make you look more approachable, but also more of a target for the enemy.' And Naruto would probably pout in response. Also, Hinata might be really shy around Naruto now, blushing more than usual because she finds his cat - like features kind of cute.
One could be 'The Fox - Transformed Harry'. In this story, Harry might get his fox ears and tail due to a magical spell gone wrong. It could be a journey of him trying to reverse the spell while also dealing with the new physical features and how they affect his daily life at Hogwarts.
Maybe there's a magic spell gone wrong in Paris, and Marinette is the only one affected in this unique way. She then has to deal with the attention she gets, both positive and negative, from her classmates. Chat Noir might be intrigued and offer to help her understand her new feline side, leading to some cute and comical interactions.
You can start by looking on popular fanfiction websites like Archive of Our Own (AO3). There are often tags related to Marinette and her having cat ears and a tail. Just search using those tags and you'll likely find some interesting stories.
Well, some Tumblr blogs might also feature such fanfiction. There are a lot of Harry Potter - themed Tumblr blogs out there. You could search for relevant tags like 'harry has cat ears' or something related to it. However, the content might be a bit scattered, so it might take some time to find exactly what you're looking for.
One great one could be 'The Feline Adventures of Fem Harry'. It has an interesting plot where Fem Harry uses her cat form to explore Hogwarts in ways she couldn't as a human. The descriptions of her cat behavior are really cute and it adds a new dimension to the Harry Potter world.
I recommend the novel "Library of All Things" to you. The male protagonist, Su Nian, accidentally obtained a library system that allowed him to travel through different time and space. As a result, he went to the immortal world to cultivate. Sometimes, he would grow cat tails and cat ears. The female lead worked in the library, but she had transmigrated. I hope you like this fairy's recommendation. Muah ~😗