There could be a group of snake - hunters in the fanfiction. They see Harry's relationship with the snake as a threat to their own interests, which might be to hunt snakes for their valuable magical parts. They would then try to stop Harry from communicating with the snake and might even try to harm him in the process.
It could be a dark wizard who also has the ability to communicate with snakes and wants to use this power for evil. He might be jealous of Harry's connection with the snake and try to disrupt it or steal the snake for his own dark purposes.
Maybe the snake itself turns out to be the antagonist. It could be deceiving Harry all along, pretending to be friendly but actually leading him into a trap. It could be working for a greater evil, like an ancient dark force that wants to destroy Hogwarts or the entire wizarding world.
The main antagonist could be a Slytherin student who has a vendetta against Harry. When they see Harry with the snake, which they believe should belong to a Slytherin, they start causing trouble for Harry. They might try to frame Harry for using dark magic with the snake or try to steal the snake to prove their superiority over Harry.
It could be a dark wizard who also has an interest in the snake. Maybe he wants to use the snake's powers for evil purposes, like opening a portal to a dark dimension or controlling other magical creatures. So he becomes the main obstacle for Harry as he tries to protect the snake.
The antagonist could be a jealous Slytherin student. They might see Harry's new snake as a threat to their own status in Slytherin house. So they try to steal the snake or harm it in some way.
It could be a jealous Slytherin student. Since snakes are often associated with Slytherin, this student might feel that Harry has no right to take care of a snake. They could try to steal the snake or cause trouble for Harry, like spreading rumors that the snake is dangerous and should be removed from Hogwarts.
It could be the person who sent the venomous snake after Harry Potter. Maybe a dark wizard who wants to harm Harry.
There could be a jealous student. This student might be envious of Harry getting all the attention with his snake - related adventures. So, they would act as an antagonist by trying to undermine Harry's efforts in the snake fanfiction world. They could spread false rumors about snakes being dangerous or Harry's intentions being dark, causing trouble for Harry among his peers and teachers.
One possible plot could be that Harry discovers a new ability to communicate with snakes on a deeper level. Maybe he uses this to uncover a hidden secret in Hogwarts, like a secret passage that was protected by snake - based magic. The snake could lead him to an ancient artifact or some long - forgotten knowledge.
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.
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.