It's possible that a group of mischievous poltergeists or magical creatures could be the antagonists. They might play a prank on Harry that goes wrong and turns him into a ferret. And then they continue to torment him in that form, making it difficult for him to get help or find a way to transform back.
It could be Draco Malfoy. Maybe he somehow manages to use a new spell or a dark object to turn Harry into a ferret out of spite. He might then try to keep Harry in that form to humiliate him in front of the whole school.
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.
It could be Voldemort. He might see turning Harry into an elf as a way to weaken him. Since elves are generally seen as lower in the wizarding hierarchy, Harry in this form would be less of a threat to Voldemort in the eyes of many wizards.
Perhaps there's a plotline where the transformation into a ferret has some unforeseen side effects. For example, Harry might gain some ferret - like abilities such as enhanced sense of smell or speed, which he then uses to his advantage in solving a mystery or escaping from a dangerous situation in Hogwarts. He could also struggle with some of the more ferret - like instincts, like being tempted to hoard shiny objects.
It could be an old rival of the Malfoy family. Maybe a wizard whose family was wronged by the Malfoys in the past. This wizard has been biding his time, waiting for the perfect opportunity to strike. Turning Draco into a baby is his way of getting revenge on the family, making them look weak and powerless.
The antagonist could be an evil wizard. He might be jealous of the oc's natural magical abilities and decides to turn the oc into a kitten to weaken them. Another option is a vengeful fairy. Maybe the oc accidentally stepped on her flower garden, and in retaliation, she uses her magic to transform the oc into a kitten. Or it could be a power - hungry sorceress who wants to use the oc's new kitten form for some dark magic experiment.
There might be a jealous friend or acquaintance who feels that Stiles, as a girl, is getting too much attention. This person could then start spreading rumors or causing trouble for Stiles in an attempt to bring her down.
It could be a dark wizard who has a long - standing vendetta against Ron's family. This wizard might see turning Ron undead as a way to get revenge on the Weasleys. He could be lurking in the shadows, trying to make Ron's undead state even worse by casting more dark spells on him.
Perhaps a powerful sorceress. She could be jealous of Robin's connection to Batman and the other heroes, so she uses her magic to turn Robin into a girl as a way to humiliate and weaken him. The sorceress could then try to use Robin's new form to her advantage, like trying to make Robin turn against the heroes or something.
It could be Team Rocket as always. Since Ash is now a Pikachu, they might see it as an easy target to capture. They could have new gadgets specifically designed to catch Pikachu - Ash, and constantly try to get their hands on him throughout the fanfiction.
Another possibility is that the main antagonist is an evil scientist who was responsible for Ash's transformation in the first place. This scientist could be trying to create some sort of chaos in the Pokémon world by turning a well - known trainer like Ash into a girl. And now, Ash as a girl has to stop this scientist from causing more harm to the Pokémon world.