Perhaps a group of poachers in the wizarding world who hunt magical animals could be the antagonists. If Harry is living as a rare magical animal, they would be after him. They might not know his true identity but would be attracted to the magical properties he has in his animal form. Harry would then have to use his wits and any remaining magical abilities to avoid being caught by them.
It could be a dark wizard who wants to take over the toy house for its magical properties. Maybe he believes that there are powerful artifacts hidden within the toys that can make him invincible.
It could be a Frost - Witch. She might be jealous of Harry's connection to magic from Hogwarts and wants to use his powers for her own evil plans in the frost - covered land.
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 possible plot could be that Harry is turned into an animal, like a stag, which is his Patronus. He then has to navigate the wizarding world in this new form. He might use his animal instincts to discover hidden secrets or evade his enemies. For example, he could sneak into places unnoticed as an animal, which he couldn't do as a human. He might also communicate with other magical animals in a way he never could before, and they could help him on his quests.
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.
A new dark wizard or witch could be the main antagonist. This character, jealous of Harry's fame and power, creates a situation where Harry is framed for a heinous crime. They then ensure his arrest so that they can take over the role of the 'chosen one' in the eyes of the wizarding public and start spreading their own brand of dark magic.