Who could be the main antagonist in a 'Harry Potter turned into a merman' fanfiction?

A power - hungry merman who wants to rule over all the mer - people. He sees Harry as a threat because Harry has some special powers or a claim to the throne that he doesn't want to share.

It could be Voldemort. He might see Harry's new snake form as a way to either use him for his own evil purposes or as a threat to his power. Since snakes are often associated with Slytherin and dark magic, Voldemort might try to control Harry.

It could be Voldemort. He might see turning Harry into an elf as a way to weaken him. Since elves are generally seen as lower in the wizarding hierarchy, Harry in this form would be less of a threat to Voldemort in the eyes of many wizards.

Well, it could start with Harry being cursed by a dark spell during one of his adventures near the Black Lake. Suddenly, he starts to feel strange sensations and discovers his legs are fusing together and turning into a merman's tail. He has to adapt to this new form. Maybe he discovers he can communicate with the merfolk in a different way now and they have some secrets about the wizarding world that only he can learn in this form.

Perhaps the main antagonist could be a dark magic user who was the one responsible for turning Harry into a girl in the first place. Their goal might be to disrupt Harry's life and his role in the fight against the dark forces. This antagonist could continue to create obstacles for Harry as a girl, like trying to prevent him from getting the help he needs to change back or causing trouble for his friends who are trying to support him.

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.

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.

Voldemort could still be the main antagonist. Even if Demetri has some sort of connection or change in the fanfiction, Voldemort's overarching evil plans would still be a major threat to both Harry and Demetri, especially if Demetri tries to deviate from the Death Eaters' path.

The minotaur itself could be the main antagonist. It's a powerful and dangerous creature, and if it's been set loose in Hogwarts or is guarding something that Harry needs, it would be a major obstacle for him to overcome.

The main antagonist could be a dark wizard who has made the graveyard his lair. He uses the dark energy from the tombstones and the restless spirits there to power his dark magic. Harry stumbles upon this wizard's activities and has to stop him.