Dataminers have discovered a whole bunch of Galarian Pokémon in Pokémon GO's code that are expected to debut during this coming week's final Throwback event.
Pokémon first introduced fans to different versions of monsters they were already familiar with when they created the Alolan region. We didn't only see pre-existing Pokémom that looked different, but also the same Pokémon with different types to their original forms. The surroundings in which they live is the reason for their differing appearance and powers.
This happened again in 2019 when the release of Sword & Shield welcomed Pokémon players to the Galar region. Previously created Pokémon with different forms appeared, which were brought about by the harsher climates of the Galar region. (It is based on the UK after all.) A couple of Galar forms debuted in Pokémon GO to celebrate the release of Sword & Shield, and now even more are coming to the mobile game.
Niantic has revealed a few of the Galar versions coming to Pokémon GO, as well as how and when they will be available to trainers. Galairian Meowth and Stunfisk encounters will be rewards for completing certain research tasks during the Throwback Challenge Champion 2020 Event. They will also hatch from 7 km eggs during this time along with Galarian Zigzagoon and Darumaka.
As for their evolved forms, since Galarian Meowth doesn't evolve into Persian (it evolves into a new Gen 8 Pokémon, Perrserker), players will not be able to evolve it for the time being in Pokémon GO. However, according to dataminers, evolved versions of Zigzagoon and Darumaka will be added to the Pokémon GO Pokédex. That includes Linoone, Damnaitan, and Darmanitan Zen Mode.
That's not all dataminers have discovered in Pokémon GO's code, either. There are a few other Galarian variations of Pokémon not yet announced by Niantic. The assumption is that they will also be added during the upcoming Throwback Challenge Champion Event. They include Galarian Ponyta, Farfetch'd, and Slowpoke. Ponyta's evolution Rapidash will also be added to the game, but since Galarian Slowbro and Slowking don't yet exist in Sword and Shield, Galarian Slowpoke will not be able to evolve. At least not yet.
Source: Pokémon GO Hub