https://www.gamespot.com/articles/pokemon-go-adds-two-new-gen-3-legendaries/1100-6457851/

Lugia may be leaving Pokemon Go very soon, but two more Legendary Pokemon are set to take its place. Beginning today, April 2, players around the world will be able to encounter Latios and Latias, the twin Eon Pokemon from Ruby and Sapphire.

Both Legendaries arrive in the game simultaneously, but each one can only be found in a different part of the world for a limited time. From now until May 8, Latios can be encountered in North America, South America, and Africa, while Latias will appear across Europe and Asia. On that date, the two Pokemon will swap regions, where they will remain until June 5.