Niantic has announced that the next wave of new Pokémon to be added to Pokémon Go will not be generation four Pokémon from Sinnoh, but the Alolan Pokémon from the generation seven Pokémon Ultra Sun and Moon games.
Alolan Form Pokémon are different variants of classic Kanto Pokemon that were introduced in Pokémon Sun and Moon. These Pokémon look different to their original counterparts and come with their own unique typing and movesets.
Niantic up until now had been releasing Pokémon from each generation of Pokémon games in order with Kanto, Johto, and Hoenn Pokémon currently available to capture. Sinnoh was expected by many to be next series of Pokemon to be released until Niantic decided to skip ahead to the recent generation.
Most of the Alolan Pokémon are just different evolutions to pre-existing Kanto Pokémon, so it will be interesting to see how Niantic implements Alolan Pokémon into the game.
One way that regular Kanto Pokemon could evolve into their Alolan Counterparts could come from the game's weather mechanic, as these Pokémon evolved differently due to the tropical climate of the Alolan region. One theory could be that Pokémon will evolve into their Alolan forms on warm and sunny days, or only during the Summer.
There is currently no date on when trainers will be able to get their hands on Alolan Pokémon—but with Summer coming up, fans hopefully won't have to wait long to hear more news.