It could be another wizard who wants to use the dragon for evil purposes. This wizard might try to stop Harry from taming the dragon so that he can have it for himself.
There could be a dark magical creature that has some sort of control over the dragon. This creature doesn't want Harry to tame the dragon as it would lose its influence. So it creates all sorts of obstacles for Harry during his taming process.
Maybe Harry uses his unique magic skills to communicate with the dragon. He could start by showing kindness and respect, like offering it food. Then gradually gain the dragon's trust and finally tame it.
There could be a group of pure - blood wizards who believe that dragons are beneath them and that Harry is tainting the wizarding world by associating with them. They would try to stop Harry from his dragon - related activities, and they might use underhanded tactics like casting spells on him or trying to get him expelled from Hogwarts.
Maybe a group of dark wizards who are jealous of the power of the dragon knights. They form an alliance to destroy the dragon knights and anyone associated with them, making Harry and his newfound friends their main targets.
It could be Voldemort still. Even with Harry having the power of a dragon god, Voldemort would be a major threat. He might be envious of Harry's new power and try to steal it for himself.
In some cases, it could be a jealous Hogwarts student. This student might be envious of Harry's special bond with the dragon. They could try to undermine Harry and the dragon's relationship, maybe by spreading false rumors or trying to get the dragon in trouble with the school authorities. This student could be working with some dark forces indirectly, as their envy makes them vulnerable to being influenced by the wrong side.
Perhaps a group of anti - dragon fanatics within the wizarding world could be the main antagonists. They see the dragon as a dangerous and uncontrollable creature and will do anything to get rid of it, even if it means going against Harry.
Maybe it's an organization that believes the dragon's blessing is an abomination to the wizarding world's natural order. They see Harry as a threat and will stop at nothing to eliminate him and the power he has received. They could be a secret group within the Ministry of Magic that has its own radical ideas about magic and purity.
It could be a dark wizard who wants to control the holy dragon for his own evil purposes. He might see the dragon as a weapon to take over the wizarding world.
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.