Obviously, Draco Malfoy is the central character since he's the one who turns into a ferret. Harry Potter often appears as well, as he has a history of conflict with Draco. Hermione Granger might also be there, perhaps using her knowledge to try and figure out a way to turn Draco back. Ron Weasley could be included too, probably making some snide remarks about Draco's new form.
Well, in some 'Draco turned into a ferret fanfiction', there could be a plot where Draco in ferret form discovers new things about the Hogwarts grounds that he never noticed as a human. Maybe he stumbles upon a secret passage or a hidden magical creature. Also, there could be a story where his transformation is a result of a misfired spell by a student, and the whole school has to figure out how to reverse it.
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.
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.
Perhaps it's a prank gone wrong. A group of Gryffindors, thinking they could teach Draco a lesson, cast a spell on him that they thought would just embarrass him. But instead, it turns him into a baby. Now, Hermione, being the brilliant witch she is, feels responsible for finding a cure. She starts researching in the library, while Ron is left to keep an eye on baby Draco. Ron has no idea how to take care of a baby, and there are lots of comical moments as he tries to feed Draco and keep him from crying.
Well, at first, Harry is just doing his duty to take care of Draco in his ferret form. But as time passes, they start to communicate in a different way. Draco, unable to be his usual snarky self as a ferret, has to rely on Harry. Harry realizes that Draco isn't all bad, and Draco, in turn, sees Harry's kindness. So, their relationship evolves into a more friendly and trusting one.
One idea could be that in this fanfiction, Draco's transformation into a girl might be due to a magical accident. Maybe a spell backfired while he was trying to outdo Harry Potter in a duel. This could lead to a whole new set of adventures as he has to navigate Hogwarts in a different body. He might face new challenges with his Slytherin peers, who may or may not accept him in this new form.
One interesting plotline could be that Harry discovers Draco in his ferret form injured in the Forbidden Forest. He decides to take care of him secretly in his dormitory. As he nurses Draco back to health, they start to understand each other better, breaking down the long - held grudges between them.
Perhaps the cause of Kagome becoming a child in the fanfiction could be a cursed spell from a powerful demon. This demon might have wanted to weaken Kagome's powers as she was a threat. As a child, Kagome could have some child - like innocence and different abilities. For example, she might be able to communicate with certain spirits more easily because children are often seen as more pure in such fictional settings.