Ash would be terrified. He is facing a group of Spearow without his most reliable Pokémon by his side. He would also feel disappointed in Pikachu. After all the adventures they've had together, he never expected Pikachu to leave him in such a dangerous situation. He might start to doubt their friendship and partnership.
Perhaps Pikachu is lured away by something more appealing. There could be a rare and powerful Pokémon or a special kind of food that distracts Pikachu from protecting Ash. Also, Pikachu might have received some false information. For example, it might have been tricked into thinking that Ash has a better chance of escaping the Spearow without it. In the heat of the moment, Pikachu makes a hasty decision based on this wrong information.
I'm not sure if there is a specific 'ash no pikachu fanfic' out there. There are countless fanfics in the Pokémon fandom, but this particular one might be hard to find. You could try searching on dedicated fanfic websites like FanFiction.net or Archive of Our Own and use the relevant keywords to see if it exists.
Definitely. The idea of Ash and Pikachu's friendship is very popular in fanfiction. Their relationship is iconic. Fanfictions could tell stories of how they first met and how that initial connection grew into an unbreakable bond. It could also include side characters from the Pokémon world who notice and are inspired by their friendship, like Misty or Brock seeing how Ash and Pikachu always have each other's backs.
In this kind of fanfiction, Ash might find it really hard at first. He's used to being a human trainer, and suddenly becoming a Pikachu would be a shock. He'd have to learn how to control Pikachu's electric powers which he only used to command before.
In a fanfiction, Ash and Pikachu could enter a Pokemon contest where the theme is lemons. Pikachu has to use its skills in a creative way related to lemons. Ash trains Pikachu hard, and they come up with a unique performance where Pikachu zaps lemons to create a spectacular light show. This earns them high marks from the judges and they make new friends among the other contestants.
One possible reason could be that Ash senses a dangerous situation where Pikachu's full power might attract unwanted attention or put them in more danger. For example, if there are powerful and unseen enemies nearby, Ash might hold Pikachu back to keep a low profile.
One possible reason for Pikachu leaving Ash in fanfiction could be a sense of self - discovery. Maybe Pikachu feels it has reached a point where it needs to explore the Pokémon world on its own terms, without Ash. It could be seeking new challenges or to find its true place among other Pokémon.
Well, if Ash turns into a Pikachu in a fanfiction, it would be quite a transformation. Firstly, his communication with Pikachu would become more of an internal dialogue. And he'd have to get used to moving around on all fours and using Pikachu - like movements. His battles would be completely different too. He'd be relying on Pikachu's natural skills rather than his own battle strategies as a human trainer.
One popular 'Ash and Pikachu fanfic' could be about their adventures in a new, uncharted region filled with unique Pokémon. It might involve them facing challenges that test their bond in ways they haven't before.