Perhaps a group of Frost - Elves who have turned to the dark side. They might see Harry as an intruder in their frosty domain and try to stop him from leaving or from uncovering their secrets. They could use their knowledge of the frost - covered terrain and their own magical abilities to create obstacles for Harry.
A powerful Ice - Golem could be the main antagonist. It might be under the control of a dark wizard who wants to keep Harry trapped in the frost world. The Ice - Golem would be very strong and hard to defeat, constantly causing trouble for Harry as he tries to find his way out.
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.
One possible plot for this fanfiction could be that Harry Potter, as a child, is lured into a world of frost by a mysterious force. This world could be a parallel dimension. He has to figure out how to get back to his own world while also dealing with the challenges of the frost world. He might have to learn new spells or magic that are specific to the cold environment. He could also uncover some long - hidden secrets about his family or Hogwarts that are somehow related to this frost world.
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 distant relative of Voldemort who has been biding their time in the shadows. They might have a vendetta against the Potter family for what happened to Voldemort. This relative could be secretly gathering followers and planning to strike at the second child when they least expect it.
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 new dark wizard who is jealous of Harry Potter's fame and success. This dark wizard targets Harry's child, thinking that by causing trouble for the child, they can somehow diminish Harry's reputation in the wizarding world. He might try to frame the child for various dark magic incidents around Hogwarts.
It could be a descendant of one of the Death Eaters who holds a grudge against the Potter family. They might try to finish what Voldemort started and target Albus specifically.
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.