It could be Dumbledore. In this fanfiction, Dumbledore might see Harry making a Horcrux as a threat to the wizarding world and tries to stop him at all costs. He could be the main antagonist because he stands in the way of Harry's actions, even if Harry has what he believes are good reasons for making the Horcrux.
The Ministry of Magic could be the main antagonist. If they find out that Harry is making a Horcrux, they would view it as a huge threat. They might send Aurors after Harry and try to stop him through any means necessary, even if Harry has a different intention behind making the Horcrux, like trying to use it to defeat a greater evil.
It could be Voldemort still. Since Harry is dabbling in dark magic by making a Horcrux, Voldemort might sense this and try to use it to his advantage. He could either try to steal Harry's Horcrux or manipulate Harry further into the dark side.
It could be a dark wizard who wants to capture Pikachu for its powerful electric abilities. He might plan to use Pikachu's electricity to power some sort of dark magic device.
Well, one possibility could be that Harry is forced into a dark situation where he believes making a Horcrux is the only way to save his friends. Maybe Voldemort has captured Hermione and Ron and threatens to kill them unless Harry does something drastic. So, in a moment of desperation, Harry delves into the dark magic of Horcrux - making, but he struggles with the moral implications throughout the story.
If Harry decides not to go to Hogwarts, the main antagonist could be the Ministry of Magic. They might see Harry as a threat since he's trying to practice magic on his own without proper supervision. They could send Aurors after him to stop him from causing any chaos in the Muggle world or from becoming a powerful, unregulated wizard.
Obviously, Harry Potter is one of the main characters since he is the one who is a Horcrux. His friends Ron and Hermione are likely to be main characters as well, as they are always by Harry's side in the original story and would play important roles in dealing with this new situation.
If Harry Potter makes a Horcrux in fanfiction, it would be a major departure from his character. Horcruxes are dark magic, and Harry is firmly on the side of good. One consequence could be that he starts to be corrupted by the dark magic within the Horcrux. His friends might turn against him as they see him slipping into darkness. He could also lose his connection to the positive magic that has always defined him, like his ability to use the Patronus charm.
A power - hungry merman who wants to rule over all the mer - people. He sees Harry as a threat because Harry has some special powers or a claim to the throne that he doesn't want to share.
A new dark wizard who has emerged from the shadows, someone who idolized Voldemort and is trying to pick up where he left off. This antagonist could be using new and more insidious magic that Harry has to figure out how to combat as an Auror.
A power - hungry noble could be the main antagonist. This person might be jealous of the Queen's position and wants to overthrow her, and sees Harry as an obstacle in his way.
Perhaps Voldemort himself, even though he doesn't have the natural ability to shapeshift in the original story. In the fanfiction, he could find a way to mimic shapeshifting, or he could use dark magic to control other shapeshifters to go against Harry. This would make for an even more dangerous and complex conflict as Harry has to face his greatest enemy with this new aspect of his powers.