I'm not sure if there is such a specific fanfiction. Fanfiction can cover a wide range of themes and scenarios, but this one seems rather unusual. It might be difficult to find, and also, some platforms may not allow certain types of content if it violates their rules.
If it exists, it might involve a complex and rather unorthodox relationship dynamic. May could be the source of some form of harm to Ash and Gary in a situation where they are in a male - pregnancy state, which is very different from the normal Pokemon storylines. However, this is all speculation as it's not a common or typical Pokemon - related concept.
Ash mpreg fanfiction is about the character Ash in a situation of male pregnancy in fan - created stories. Fans who write such fanfictions might be inspired by their love for the original source material. They take the concept of Ash and add this unexpected and often very creative element of mpreg to it, which can range from sweet and emotional stories to more adventurous ones.
Pikachu is a very likely candidate. It's Ash's most iconic Pokemon and has a deep bond with him. Whenever Ash is in trouble, Pikachu often steps up to protect him.
It's hard to pinpoint exact popular 'ash mpreg fanfiction' works. Different fandoms have different levels of activity in this type of fanfiction. However, if you look at popular fanfiction websites and search for 'ash mpreg' along with the name of the specific fandom Ash belongs to (if known), you might find some works that have a relatively high number of views or reviews, which could indicate their popularity among fans interested in this type of content.
Ash could be a Pokémon trainer with a different starting point in this fanfiction. Born in 1987, by 1997 he might have already made a name for himself in local Pokémon battles. He could have a team of Pokémon that he rescued from difficult situations, like a Charmander that was abandoned by its trainer. His story could revolve around giving these Pokémon a second chance and proving that Pokémon don't need to be 'perfect' to be strong.
The impact of Ash's loss in the fanfiction could extend beyond just himself. His Pokémon might also feel demoralized. However, this could also be an opportunity for them to grow stronger together. Ash could start training more intensively, seek new training methods, or even try to catch new Pokémon to strengthen his team.
In a fanfiction about Ash born in 1987 and his situation in 1997, we could see an alternate story where Ash is not the typical rookie trainer. Since he's a bit older than in the original, he might be more knowledgeable about Pokémon lore. He could be on a quest to uncover some ancient Pokémon secrets in 1997. He might travel to different regions, not just for battles but also to study old Pokémon texts and artifacts, and along the way, form an unusual team of Pokémon that are attracted to his in - depth knowledge.
There are also fanfictions where Ash catches Rayquaza. This could be set during a time when the sky is in chaos due to some dark energy. Rayquaza is drawn to Ash because of his pure heart and determination. Ash, with his usual courage, teams up with his pokemon to assist Rayquaza in restoring balance. As a result, Rayquaza allows Ash to catch it.
If we consider the common growth pattern in the Pokemon series, Charmander evolving into Charmeleon could be an early evolution in the fanfiction. Since Charmander has a clear evolution line and Ash has a strong bond with it, it would make sense for it to be one of the first to evolve, perhaps due to a tough battle or a special encounter that accelerates its growth.