https://www.gamespot.com/articles/six-new-shiny-johto-pokemon-added-to-pokemon-go/1100-6461293/

Pokemon Go adds six new shiny Pokemon for trainers to catch. In celebration of the mobile game's limited-time Johto Festival event, which increases the spawn rate of all Johto exclusive Pokemon, the six additions are all part of the Gen 2 Pokedex.

In a tweet, Pokemon Go revealed both shiny Natu and Sunkern had been added, while hinting there might be more than just the two. It's since been revealed that the third shiny addition is Pineco, and that all three Pokemon evolve into shiny versions of Xatu, Sunflora, and Forretress respectfully.