In Hocus Pokémon, Ash and friends encounter a witch named Lily who claims to be able to make a spell that allows the user to think like a Pokémon. Ash, Brock, and Misty all try to help her complete the spell. At the end of the episode, the spell is put onto Ash, but instead of literally being able to read a Pokémon's mind (as predicted), the spell turned him into a Pikachu because of an accident on Lily's behalf. The spell was not permanent, and Brock noted that it did what Lily intended; Ash was thinking exactly like a Pikachu, as he had the mind of one. While transformed, Ash's hat, eyes, voice, and hair style remained the same.
While the term "Ashachu" is mainly used in relation to his transformation in Hocus Pokémon, it is often brought up in hopes for future possibilities, as Ash changed back into his normal appearance at the beginning of the following episode.
Although he does not appear in the games or reappear in the anime, a similar Pikachu variant set was distributed for I Choose You!, which are ordinary Pikachu wearing the various hats that belonged to Ash in each anime series.