https://www.gamespot.com/articles/pokemon-go-getting-alolan-forms-of-gen-1-pokemon/1100-6459086/

A variety of new Pokemon will soon begin appearing in Pokemon Go. Developer Niantic announced that Alolan forms of classic Gen 1 monsters are on the way to the popular mobile game, and they'll begin popping up around the world just in time for summer.

"Whether your forecast calls for rain or shine, we're celebrating all around the world by introducing some special Pokemon from the tropical Alola region to Pokemon Go," Niantic said on the official Pokemon Go website. "Get ready for some of the Pokemon originally discovered in the Kanto region to appear in their Alolan forms!"