He could find that his relationship with his Pokemon changes. Some Pokemon might be more protective of him now that he's in a more vulnerable form. Others might see him as less of a trainer and more of an equal. Ash would also have to deal with how the Pokemon world views hedgehogs. Are they seen as pests or something more interesting? This could lead to different reactions from different Pokemon species. And as he travels, he might discover new places that are more suitable for a hedgehog - like small burrows or areas with lots of bushes where he can hide and rest.
The Pokemon that have been with Ash for a long time, like Snorlax, might have a deeper connection with him and be able to sense that it's still Ash, even though he looks like a hedgehog. Snorlax might be more accepting right away and try to protect Ash from any potential danger. On the other hand, the newer members of his team might be more likely to question whether this hedgehog - like Ash is still their rightful trainer. But as time passes, they would all start to realize that Ash's spirit and his bond with them haven't changed, despite his physical transformation.
Well, he might start to see the world from a Pikachu's perspective. For example, he would experience the small size and how everything looks so huge. He could also find it difficult to communicate with his human friends in the normal way since he is now a Pikachu. Maybe he would have to rely on Pikachu's ways of expressing emotions like using electrical signals.
The way Ash adapts to Pikachu's instincts could be really interesting. He might find himself being more drawn to shiny objects or having an urge to store food in a certain way. And his relationship with his own Pikachu (if it still exists in the story) would be very different. They could become like equal partners instead of trainer - Pokémon.
In a fanfiction where Ash turns into a hedgehog, it could be a wild ride. Firstly, his appearance would be completely different. He'd be small and furry with spines. This could make it difficult for him to travel as he used to. His clothing would probably be all wrong for his new form too. Regarding his Pokémon battles, he'd have to come up with new strategies. Instead of standing on the sidelines and giving commands, he might be able to directly participate in a more physical way, using his hedgehog body to dodge attacks or even launch small counter - attacks. It would definitely add a whole new dimension to the Pokémon world in that fanfiction.
Maybe it's a legendary Pokemon in disguise. In some fanfictions, legendary Pokemon often play important roles. It could give Ash the egg as a test or because it senses that Ash has a pure heart. Another possibility is that it could be a young trainer who can't take care of the egg anymore and trusts Ash to do a good job.
Misty is a very likely candidate. She has been with Ash from the early days of his journey. They have a deep bond and have shared many adventures together. Their relationship has always had a bit of a romantic undertone, so in a fanfiction, it would make perfect sense for them to get married.
I'm not aware of any non - inappropriate 'Ash gets a Pokemon pregnant' fanfiction. This concept is not in line with the family - friendly nature of the Pokemon franchise. Fanfictions usually revolve around themes like battles, training, and the bonds between trainers and Pokemon. A concept like this is quite abnormal and would not be easily found in the realm of proper fan - created works.
One possible plot could be that Ash wakes up one day as a girl and has to deal with all the changes in his daily Pokémon adventures. For example, he might have to adapt to new clothing styles and different social interactions. His Pokémon might also react in various ways to his new form.
In this fanfiction, it could be that Ash's marriage is a way to bring peace between different regions. He marries a princess or a powerful trainer from a rival region. Their union symbolizes the unity of Pokémon trainers everywhere. The story could then detail the political and social implications of their marriage, as well as the more personal aspects like their relationship building and how they learn to love and respect each other's Pokémon.