It could be a rogue wizard who wants to use Harry's new merman form for his own evil purposes. Maybe he wants to study Harry's transformation to find a way to create an army of half - human, half - merman creatures to take over the wizarding world and the underwater realm.
Perhaps the main antagonist is an old enemy of Dumbledore who has discovered Harry's transformation. He believes that by controlling Harry in his merman form, he can finally get revenge on Dumbledore. So he sends his minions to capture Harry and keep him trapped in the underwater world, away from his friends at Hogwarts who could potentially help him break the spell.
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.
It could be Malfoy. He might make fun of Harry for wearing a dress and try to use it to humiliate him in front of the whole school, which would then lead to a conflict between them that goes beyond the usual rivalry.
Perhaps a new dark wizard or witch who has infiltrated the Ministry of Magic and made sure Harry was sent to Azkaban. This antagonist could be working towards a bigger plan to take over the wizarding world and sees Harry as a major threat even when he's locked up.
Maybe a dark wizard who has an interest in Harry's newfound inheritance. This antagonist could want to steal his powers or use him for their own evil purposes. They might see Harry as a key to unlocking some ancient and powerful magic that they've been seeking.
Perhaps a group of dark wizards who think that by disrupting time, they can bring back the dark ages of wizardry. They target the Timekeeper as the only one who can stop them. They use dark spells and curses to try and stop the Timekeeper from fixing the time - related problems they create.
It could be Voldemort. He might see Harry's new waterbending ability as a threat and try to find a way to counter it. Maybe he would seek out dark magic related to fire to oppose Harry's water powers.