Maybe Harry discovers some dark family secrets that make him question his place on the 'good' side. He could find out that his ancestors had strong ties to dark magic, and this knowledge could slowly corrupt him. He might start to experiment with dark spells, thinking that he can control them and use them for what he believes is the greater good, but gradually slips towards the darkside.
It could still be Voldemort. Even if Harry is turning to the darkside, Voldemort might see him as a threat because Harry has his own unique power. Voldemort might not want to share his control over the dark forces with Harry, so he would become the main antagonist in a fight for dominance in the dark world.
Maybe Chat Noir is betrayed by his friends. This could push him to the dark side. He might start to question his values and the side he has been fighting for all along.
Another possibility is that Ladybug could become the main antagonist in a sense. If Chat Noir turns dark and Ladybug tries to stop him, from Chat Noir's new perspective, Ladybug could be seen as the one standing in his way. Their relationship would be completely flipped, and Ladybug would have to find a way to bring Chat Noir back to the light while also dealing with his attacks.
In such a fanfiction, Harry Potter's transformation into a horse could be a part of a larger plot. For example, it could be related to a dark magic experiment that was meant for something else but accidentally affected Harry. His friends, like Hermione and Ron, would be in a frenzy trying to figure out how to turn him back. Hermione would likely be researching every book she could find on Transfiguration and dark magic. Ron, on the other hand, might be more focused on keeping Harry safe and comfortable in his new form. And Harry himself would be dealing with the shock and confusion of being a horse, all while still having his human mind intact and trying to communicate with his friends in whatever limited ways he can.
One possibility is that Harry Potter becomes a hybrid of a wizard and a centaur. He would inherit the centaur's knowledge of the stars and nature. His lower body being that of a centaur would make it difficult for him to move around Hogwarts in the normal way. He would have to find new ways to get to his classes. And he might be shunned by some students who are afraid of his new and strange appearance, but he could also form new alliances with those who are more open - minded and interested in his unique situation.
He might start spending all his free time watching animations and collecting related merchandise. He could even try to use magic to create his own animated world.
Well, it could be really interesting. Maybe Harry would start to see the magical world through the lens of cartoon logic. He might use cartoon - like spells that have over - the - top effects. For example, instead of a normal Patronus charm, his Patronus could turn into a huge, colorful, and goofy - looking animal that acts like a cartoon character, bouncing around and making silly noises.
Well, he would lose his human form and all the abilities that come with it. He would have to learn to move and hunt as a basilisk, which would be a huge adjustment. His friends might not recognize him and could even be afraid of him.
In some fanfiction, it might be related to a hidden magic within Hogwarts itself. There could be a mysterious force in the school that reacts to Harry's unique connection to the place. For example, if he uncovers a long - forgotten chamber or artifact that has some sort of transformative power. Since basilisks are associated with the Chamber of Secrets, it's possible that something in that area could be the cause.