https://www.gamespot.com/articles/pokemon-go-adding-gen-4-starters-from-sinnoh-regio/1100-6462380/

Your Pokedex is about get a whole lot bigger as The Pokemon Company and Niantic have announced that monsters from the Sinnoh region (including Gen 4's starters) are coming to Pokemon Go. A new trailer, which has a nice Planet Earth vibe, shows brief glimpses at a Turtwig hanging out in a grassy area, a Chimchar rousing from sleep in a cave, and a Piplup swimming beneath a layer of ice.

"A vast new world is waking up around us," the voice over says. "With more to explore than we could have ever imagined." The trailer ends with a pair of red eyes emerging and, although the Pokemon they belong to remains cloaked in darkness, it looks very much like Giratina, a Legendary from the Sinnoh region.