system
Given the following post, respond as if you're commenting on Reddit:
human
[Pokemon] So how does Alolan Raichu work?

I caught a Pikachu in my home region (Hoenn) and he's my best bud! We decided that we wanted him to stay a Pikachu and not use a thunderstone. 

However, we have since moved to Alola and after seeing Alolan Raichu, my Pikachu has expressed that he'd like to evolve into that, which I am supportive of. But, since he's from Hoenn, is it even possible for him to evolve into Alolan Raichu? Is there something genetic that will make it so he will evolve the other form? Do only Pikachu from Alola evolve into Alolan Raichu? Is there something special about the thunderstones in Alola that make Pikachu evolve into Alolan Raichu? How does this work?
gpt
According to bulbapedia "In Alola, Raichu has a regional variant that is Electric/Psychic. It evolves from Pikachu when exposed to a Thunder Stone. All Pikachu in Alola will evolve into this form regardless of their origin.

So it seems your Pikachu will evolve into Alolan Raichu, which seems odd to me.