His other Pokémon like Charizard would be completely bewildered. Charizard might initially think that this Eevee is just some random Pokémon near its territory. But when it senses something familiar about the Eevee, it would be hesitant and curious. It could try to communicate in its own way, like using small bursts of fire to get the Eevee's (Ash's) attention. The Pokémon that Ash has a closer bond with would be more determined to figure out what's going on and how to help him.
His Pokémon would have a range of reactions. Some of the smaller and more timid Pokémon might be afraid of this new form of Ash. However, the ones that are more intelligent, like Bulbasaur, would start to analyze the situation. Bulbasaur might try to use its vines to interact with Ash - the Eevee, gently prodding and sniffing to figure out what's different. Overall, they would all be concerned and try to adapt to this strange new situation with their trainer.
The plot might involve Ash turning into an Eevee and being found by a group of Eevee and their evolutions. They initially view him as an oddity since he has the memories and some of the behaviors of a human. Ash, as an Eevee, could then learn new skills from his new companions and use his knowledge from being a human trainer to help the group in various situations, such as finding food sources or avoiding predators.
A common theme is the exploration of the Pokémon world from a different perspective. As an Eevee, Ash sees things in a new light. He experiences the Pokémon world on a more intimate level, like understanding the language of Pokémon better. There's also the theme of adaptation. He has to quickly adapt to his new physical form, which includes learning to move, run, and use his new Eevee - specific abilities.
The unique character development. When a Pok mon is pregnant in a fanfic, it gives an opportunity to explore the Pok mon's personality in a new light. For example, how it copes with the physical and emotional changes. It can also show the relationship between the Pok mon and its trainer in a different way, like increased protectiveness from the trainer.
You can start by looking on fanfiction websites like FanFiction.net or Archive of Our Own. These platforms have a wide variety of Pok mon fanfics, and you can use the search function to specifically look for ones related to the 'pregnant' theme. Just be sure to read the ratings and reviews to ensure the quality.
I'm not sure of the exact name, but there's a fanfiction where Ash turns into an Eevee and gets caught by a trainer. This one is really unique as it shows the Pokémon world from the perspective of a captured Pokémon. Ash has to use his knowledge as a human - turned - Eevee to escape and also tries to communicate with the other Pokémon in the trainer's team. It's a great read for those who like a different take on the 'Ash turns into an Eevee' concept.
At first, Ash and Eevee might be a bit cautious around each other. Ash is excited to have caught a new Pokémon, but Eevee is in a new and strange environment. As they start training together, they build trust. Eevee might learn new moves from Ash's guidance, and Ash learns more about Eevee's unique abilities.
At first, his Pokémon would be startled. They've always known Ash as a human. But then, they would start to adapt. Some Pokémon, like Squirtle, might try to play with Ash in his new form, thinking it's a new kind of game. However, his more loyal Pokémon, like Greninja, would be determined to find a way to turn Ash back. Greninja might use its special bond with Ash to try and communicate with him on a deeper level in this new form, to understand what happened and how to reverse it.
His friends would be very shocked at first. Misty might be a bit worried and try to find a way to turn him back quickly. Brock, on the other hand, would be more curious and start observing Ash - as - Pikachu to see how his behavior has changed. His Pokémon friends would also be confused. Pikachu himself might be both excited to have a 'Pikachu - like' friend but also a bit sad that his trainer is in this strange state.
At first, his friends would be in disbelief. But then, they would start to adapt. Dawn might see it as an opportunity to study a Pokémon from a different perspective. Gary could be both amused and intrigued, and might offer some scientific theories on how it happened. Overall, they would all be concerned for Ash's well - being and work together to either help him get back to normal or help him thrive in his new form.
Another theme is Eevee's evolution. Many fanfictions explore different ways Eevee could evolve and how that affects its relationship with Ash and their adventures. For example, if Eevee evolves into a Vaporeon, it might lead to underwater adventures. Also, the theme of friendship and loyalty is always there. Ash and Eevee rely on each other, and this is often a central part of the stories.