Dumbledore could be the main antagonist. Harry might see Dumbledore as having used him all along for some grand plan. If Harry turns evil, he would want to break free from Dumbledore's influence. Dumbledore, with his power and wisdom, would then try to stop Harry from causing too much harm, thus becoming the main opposing force.
One possible plotline could be that Harry discovers a dark secret about his family and feels betrayed by the wizarding world. This could lead him to side with the dark forces. Another could be that he is influenced by a powerful dark wizard who manipulates his mind. Maybe he becomes envious of the power others hold and decides to take it by any means, turning evil in the process.
In Harry Potter fanfiction, the exploration of good and evil can be really interesting. Some fanfics might show how characters who seem evil in the original series have a more complex backstory that makes them not entirely bad. For example, Draco Malfoy. In some fanfictions, he could be portrayed as a victim of his family's expectations rather than a pure - bred villain.
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.
One possible plot could be that Harry is under a powerful dark spell. The transformation into a dragon might be a part of a scheme by Voldemort or his followers to either discredit Harry or use his new form for some evil purpose. Maybe Harry has to learn to control his dragon form in order to fight back and protect Hogwarts and his friends.