https://www.gamespot.com/articles/pokemon-gos-first-gen-3-legendary-is-now-available/1100-6455691/

Pokemon Go's most recent Legendary Pokemon, Ho-Oh, may no longer be available, but players didn't have to wait long for a new Legendary to replace it. Beginning December 15, players around the world will have a chance to encounter the Legendary Ground-type Groudon for a limited time.

Groudon marks the first Legendary Pokemon from Gen 3 to appear in Pokemon Go. Like previous Legendaries, it can be encountered as a Raid Battle at Gyms. You'll need to team up with other players in real-life to battle it; only after bringing Groudon down will you have a chance to capture it. Groudon will appear in the game until 1 PM PT / 4 PM ET / 9 PM GMT on January 15, 2018.