Perhaps there is a special place where the boundaries between the human world and the Pokémon world are thin. If the girls wandered into this area, the energy from the Pokémon world could start to seep into them and cause the transformation. Another idea is that there could be a genetic mutation factor. Maybe there is a hidden gene in some people that, when activated by certain environmental factors like a strange radiation or a specific type of food, can turn them into Pokémon. This would be a more scientific - based reason within the fictional context.
In a tf story, it could be due to a scientific experiment. Scientists might be trying to find a way to give humans the abilities of Pokémon. The girls could be test subjects, and something unexpected happened during the process. Also, it could be related to a curse. If they entered a forbidden area or angered a powerful Pokémon - like spirit, they could be cursed to turn into Pokémon. This would add an element of mystery and danger to the story.
There was a magical event in a small town. Some girls were at a carnival when they entered a tent that seemed to be out of place. Inside, there was a large machine with Pokémon symbols all over it. Without thinking much, the girls interacted with the machine. Suddenly, a bright light flashed. A girl with long red hair started to transform into a Charizard. Her skin became scaly and she grew wings. Another girl turned into a Bulbasaur. She had a little plant bulb on her back and her eyes became more like a Pokémon's. They were shocked at first but then they started to enjoy their new forms and the special abilities that came with it.
In some stories, it could be due to a magic spell. Just like in fairy tales, a wicked witch or an inexperienced wizard might accidentally cast a spell that changes a person's gender. Another reason could be a scientific experiment. Scientists might be trying to do something revolutionary like gender transformation through some new technology, but things go wrong and a male turns into a girl.
It might be that the man made a deal with a witch and broke the terms, causing the transformation. Or it could be a symbol of his inner transformation and represented metaphorically as a physical change into a dog. There could also be a virus or a strange potion involved.
It's really hard to say precisely without more context. 'tf' could potentially stand for 'transformation' perhaps, so it might be a story about girls transforming into horses in some fictional or symbolic way.
Magic could be the main reason. Maybe she entered a magical realm where such transformations are common, like a fairy - filled forest or an enchanted garden. For example, if she drank from a magic fountain, it could trigger the transformation.
In the fanfiction, it could be due to an experiment. Some mad scientist - like character in the Naruto universe might have been experimenting with a new form of chakra manipulation. Gaara, being in the wrong place at the wrong time, got caught in the experiment and was turned into a baby. This would then lead to a whole new set of adventures as his friends try to find a way to reverse the process.
Perhaps it's a result of the dog wandering into a place it shouldn't have, like an ancient, haunted ground. There could be some strange energy there that seeps into the dog, causing it to turn white. This change is often accompanied by a change in behavior, such as becoming more aggressive or showing signs of intelligence that are not normal for a dog, which makes it truly scary.
One reason could be for a school play or theater production. For example, if the play requires a male character to disguise as a female for the plot. Another reason might be part of a bet or a dare among friends, just for fun.
Magic could be a reason. Like in a fairytale world, a magic spell or a magic object might cause the transformation. For example, a magical amulet that when activated changes the boy's gender.
It could be due to a magical artifact. For example, if she touches an ancient and cursed dragon scale or a powerful enchanted jewel. These items often have powers that are not fully understood, and their magic can cause unexpected transformations. Also, it might be a self - sacrifice. In some stories, to save her loved ones from a dragon - related threat, the girl might choose to become a dragon herself to fight on equal terms.