One possible reason could be a dark spell cast on him by a powerful dark wizard. Maybe Voldemort's followers found a new and strange magic to use against Harry. Another reason might be related to some ancient magic that Harry accidentally triggers. For example, he might stumble upon an old, forgotten ritual site and unknowingly activate it, which results in his transformation.
Perhaps it's a side effect of a potion he was forced to drink. In the wizarding world, there are many potions with strange effects. If Harry was captured by some evil wizards and made to consume an experimental potion, it could lead to his transformation into a snake. It could also be part of a prophecy. Some prophecies in the wizarding world are very cryptic and could be misinterpreted, leading to events that seem strange like this.
One interesting idea could be exploring the psychological changes that come with the snake transformation. For example, how the character might start to view the world from a more instinctual and predatory perspective. Another idea could be the impact on relationships. Maybe friends are now seen as either threats or potential prey, which would add a lot of drama to the story.
It could be a sign of a connection to an ancient or powerful magic. In some fanfics, a character's hair turning white might indicate that they are now linked to a long - forgotten magic that has started to manifest within them. This magic could be so strong that it physically changes their appearance. For instance, they might have come into contact with an artifact that contains this magic and it starts to change them, starting with their hair.
His friends might be initially terrified. Hermione, for example, would be both scared and immediately start researching how to reverse it. Ron would probably be more shocked and not know how to react at first. Their friendship would be tested as they try to figure out what to do. They might have to keep Harry's transformation a secret from other students to protect him from being hunted or experimented on.
It could be due to a curse. Maybe someone with malicious intent put a curse on Ash that turned him into a girl.
One reason could be to explore the darker side of human nature. Even though Harry is a hero in the original series, fanfiction writers might want to see how he would be affected if he was pushed to his limits. Another reason is to create a more complex and controversial storyline. It grabs readers' attention as it goes against the normal perception of Harry.
Some fanfiction writers may have a different vision for Harry Potter's character. They might see him as more of a diplomat or a healer within the wizarding world, rather than a fighter. So, they write stories that reflect this different view of the character, where he uses his wits and other skills rather than engaging in physical or magical combat.
One reason could be that the official Harry Potter canon is so extensive and detailed that it overshadows fanfiction. The original story has such a rich world, characters, and plotlines that it becomes the main focus for many fans, leaving less room for fan - created stories.
One reason could be safety. Some fanfiction authors might think that Hogwarts is too dangerous for Harry, so they make him homeschooled to keep him out of harm's way.
One way is through distraction. The snake could draw the attention of Harry's attackers away from him. It could slither towards the bad guys and make menacing hisses, causing them to focus on the snake instead of Harry.
One reason could be a reaction to the Ministry of Magic's mismanagement. In the original story, the Ministry often made mistakes and had corrupt elements. In fanfiction, this could lead to Harry becoming an anarchist as a way to oppose such a flawed system.