https://www.gamespot.com/articles/pokemon-gos-new-ar-mode-revealed/1100-6455762/

A previous update to Pokemon Go, which is available on all devices, introduced weather to the game. You also have the opportunity to catch the Gen 3 legendary Pokemon Groudon until January 15. Dozens of other Pokemon from the series' Gen 3 games, Ruby and Sapphire, were introduced in Pokemon Go following the recent update, with the remaining third-generation Pokemon to come at a later date.