It could also be part of an experiment. He might be curious about how it feels to wear different types of clothing. Or perhaps he's been influenced by some new magical culture he's learned about that has different views on clothing and gender. There could be a spell or a potion that makes him more in touch with his feminine side, and as a result, he starts wearing dresses.
Hermione, on the other hand, could be more understanding. She might see it as Harry expressing his individuality. She could be supportive and even defend Harry if others are mean about it. In many fanfics, she's the one who tries to promote acceptance. She might say that just as witches can wear trousers, wizards can wear dresses if they like.
One possibility is that he's trying to prove a point. Maybe there's some discrimination or unfair treatment towards girls in the wizarding world that he wants to highlight. By dressing like a girl, he can put himself in their shoes, so to speak. Another reason could be that he's influenced by a new friendship or relationship. He might have a close female friend who has inspired him to try dressing like a girl, either as a form of bonding or because he admires her style.
One 'Harry Potter wears dresses' fanfic could be about Harry wearing a dress to a magical ball in Hogwarts as a form of breaking gender norms. It might show how his friends react, like Hermione being supportive and Ron being a bit confused at first.
I'm not sure about extremely well - known ones specifically focused on this concept. But in the vast world of Harry Potter fanfiction, there are likely some that have gained a following within certain circles. You could search on fanfic platforms using relevant tags like 'Harry Potter', 'dress', and 'gender - bending' to find them.
Well, perhaps in this fanfic, Harry could be forced to dress like a girl as part of a dare from the Weasley twins. They might bet him that he can't go through a whole day at Hogwarts looking like a girl without getting caught. And then there would be all sorts of comical situations as he tries to avoid detection by his friends like Ron and Hermione, who would be really surprised if they found out.
Maybe Harry could be on the run from some dark wizards and he disguises himself as a goth girl to avoid detection. He would have to learn how to act and move like a goth girl too, which could lead to some comical situations.
One possible reason could be a spell gone wrong. Maybe a prank spell intended for someone else backfired on Harry, causing him to magically transform into wearing a skirt. It could also be part of a dare in the story. His friends might have dared him to wear a skirt for a day as a part of some Gryffindor courage - testing tradition. Another reason could be for a disguise. Harry might be on a secret mission and wearing a skirt could be an unexpected disguise to throw off his enemies.
One idea could be that Harry discovers a magical dress in the Room of Requirement that gives him special powers. Another could be that he wears a dress as part of a disguise to infiltrate a wizarding event. Or perhaps he is cursed to only be able to wear dresses for a period of time and has to deal with the reactions of his friends and foes alike.
One possible plotline could be Harry exploring his identity through fashion and makeup. He might start a fashion club at school where he helps others express themselves too. This could lead to him facing some initial resistance but eventually winning over his peers with his confidence and creativity.
It could be a symbol of mystery. Harry is often in the center of many important events. The hood adds an air of mystery to his character, making others wonder what he's up to or what he's hiding. For example, when he's sneaking around Hogwarts at night, the hood might make it harder for Filch or other nosy people to recognize him.