Another option could be a group of pure - blood supremacists. They would view Harry's new hybrid form as a contamination of the wizarding bloodlines. They could band together to try and get rid of Harry, using any means necessary. They might spread rumors about him being a danger to the wizarding community, trying to turn public opinion against him so that they can carry out their plans without interference.
Maybe the Ministry of Magic could be the main antagonist. If Harry turns into a hybrid, the Ministry might see him as a dangerous abomination. They could try to capture him and keep him locked away, thinking that his new form is a threat to the wizarding world's order. They might not understand that Harry is still the same person inside and that his new form could actually be an asset in the fight against evil.
It could be Umbridge. If she discovers Harry's transformation into a veela, she might see it as something unnatural and try to use it against him. She could try to expel him from Hogwarts or make his life even more miserable than she already did in the original story.
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.
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.
Umbridge could also be a main antagonist. She is known for her strict and often cruel rules. If she discovers Harry in his female animal form, she would likely try to use it as an excuse to further oppress him. She might see it as a sign of abnormal magic and use it to justify her actions against Harry and those she doesn't like in Hogwarts.
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.
It could be a powerful vampire clan that doesn't want Harry, a new vampire, to disrupt their ways. They might try to either kill him or force him to join their ranks against his will. They could see him as an outsider who doesn't belong in the vampire world and a potential danger to their secrecy.
It could be Dumbledore. If Harry turns to the Dark Side, Dumbledore, who has always been a guiding figure for Harry, would be the main one to oppose him. Dumbledore represents the light and everything Harry is now against. Dumbledore would try to stop Harry from causing chaos and destruction in the wizarding world.
Perhaps a long - lost relative of the Potter family who feels they were unjustly excluded from inheriting the manor. This relative could be jealous of Harry's status and the wealth associated with the manor. They might use dark magic to try and claim what they think is rightfully theirs, leading to a battle of magic and loyalty within the Potter family and among Harry's friends.
It could be an old - fashioned vampire who doesn't like the idea of hybrids and sees Caroline as an abomination.
A new dark wizard or witch could be created specifically for this fanfiction. This character could be interested in using the Huntsman's skills for their own evil purposes. If the Huntsman refuses, they become enemies. This antagonist could have powers related to dark forest magic, something that the Huntsman is familiar with but in a more malevolent form.