After Ash freaked out about becoming a Pikachu, he played with his Pikachu and the other Pokémon. At the end of the episode, Ash - still a Pikachu - was rolling on the floor laughing. However, Ashchu turned back into Ash during the next episode.
He is actually Ash Ketchum in a form of Pikachu with yellow hair and ears like the regular Pikachu.
Though this was a very minor occurrence in the Anime series, it has become a target among some fans, being used in fan-fiction or fan art. Usually, Ashachu commonly makes cameos in the Pokémon series Sinnoh.