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.
A group of dark creatures that are attracted to the magic of the toy house could be the antagonists. These creatures, like dementors or boggarts, try to invade the toy house and corrupt the magic within. Harry has to protect the toy house and its inhabitants (the toys) from these evil forces.
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 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.
Perhaps some of the pure - blood wizard families who look down on house elves training a wizard like Harry could be antagonists. They might see it as a threat to the traditional wizarding hierarchy and try to stop Harry and the house elves, thinking that it's not proper for a wizard of Harry's status to be trained by 'mere' house elves.
Another possibility is a group of Hogwarts professors who are very conservative in their views. They might believe that house elves should remain in their subservient positions and would try to stop Harry from making any changes. They could use their influence within the school to make things difficult for Harry and her friends as they try to advocate for the rights of house elves.
In this fanfiction, the toy house could be a gift from Dumbledore. It has the power to shrink or grow depending on Harry's needs. Inside, the toys are actually the souls of great wizards who have been trapped. Harry has to free them by passing a series of magical tests, and in return, they teach him advanced magic that no one else at Hogwarts knows.
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.