A powerful and jealous Glaceon from a different region might be the antagonist. This Glaceon sees Ash's transformation as an abomination or a threat to the Glaceon species. So, it tries to drive Ash away or even harm him. It constantly challenges Ash in his new form and tries to make his life difficult among the Pokemon world.
It could be a Team Rocket member who has obtained a new device. This device was meant to transform Pokemon into other forms for their evil purposes. But by accident, it affects Ash instead. So, this Team Rocket member becomes the main antagonist as he tries to either capture Ash in his new Glaceon form or use him for other devious plans.
Maybe Ash gets cursed by a powerful Pokemon or a mysterious artifact. As a result, he starts to transform into a Glaceon. His friends, like Pikachu and Brock, are shocked at first. Then they try to find a way to reverse the transformation while Ash has to deal with his new form, which has different abilities and instincts compared to his human self.
Maybe Team Rocket could be the main antagonist. They might see Ash's transformation as an opportunity to finally capture Pikachu. Since Ash is in a more vulnerable state as a girl, they could try to use different tactics to take advantage of the situation.
Perhaps a jealous trainer. This trainer has always been envious of Ash's success and now that Ash is in a vulnerable state as a Zorua, the trainer sees an opportunity to get rid of him for good. The trainer could try to use his own Pokémon to capture Ash or even harm him, making the trainer a formidable antagonist in the story.
Maybe a powerful and envious Mewtwo could be the antagonist. Since Ash has now become a Mew, Mewtwo might feel threatened or think that Ash doesn't deserve such power. So, Mewtwo could try to fight Ash to prove his own superiority.
Perhaps a dark - type Pokémon that is jealous of Ash's new form. This Pokémon might see Ash as a threat and constantly try to attack him and stop him from getting used to his Latios form.
It could be a jealous trainer. This trainer might see Ash in his shiny pokemon form and want that power for himself. So he tries to capture Ash by any means necessary, setting up all kinds of traps and challenges for Ash and his friends.
Maybe a powerful and evil pokemon trainer who has a vendetta against Ash. Now that Ash is in a vulnerable pokemon form, this trainer could try to capture and control him. The trainer might have some dark motives related to using Ash's unique situation to gain power in the pokemon world.
A jealous male trainer could be the main antagonist. He might see Ash's transformation as an unfair advantage given by some mysterious power. He constantly tries to undermine her in battles and challenges, believing that a female should not be as successful as a male in the Pokemon world.
Maybe the main antagonist is an evil scientist. The scientist has been studying Pokémon transformations and when he sees Ash turn into Pikachu, he wants to experiment on him. He chases Ash around, trying to capture him with all kinds of high - tech devices. This forces Ash to use his Pikachu form's abilities and his human knowledge to outwit the scientist.
There could be a mad scientist - type character who created the situation where Ash turned into a Latias in the first place. But then, this character wants to keep Ash in that form to study him and use his new - found Pokemon abilities for their own gain. They would be the main obstacle for Ash in his journey to turn back into a human.