https://www.gamespot.com/articles/pokemon-go-adds-new-legendary-more-gen-3-pokemon-t/1100-6456656/

Niantic is rolling out a new update for Pokemon Go very soon. Beginning February 9, players will be able to catch even more Pokemon originally from Ruby and Sapphire in the mobile game, while the Legendary Pokemon Rayquaza will be available for a limited time.

From February 9 to March 16, players will be able to encounter the powerful Dragon-type Pokemon as a Raid Battle at Gyms. Rayquaza is the third Legendary Pokemon from Gen 3 thus far, following Groudon (which was available from December-January) and the current Legendary Kyogre.