https://www.gamespot.com/articles/last-chance-to-catch-this-pokemon-go-legendary/1100-6456104/

Pokemon Go's first Gen 3 Legendary, Groudon, has been a regular presence in Raid Battles since it first began appearing last month, but it won't be available for much longer. The powerful Ground Pokemon is scheduled to leave the game tomorrow, January 15, making this your last opportunity to add it to your team.

Like most of Pokemon Go's other Legendaries thus far, Groudon is an exceptionally powerful Pokemon that can only be found at Gyms. Players will need to team up with other Trainers in order to battle it, and only after Groudon has been defeated will you earn a chance to capture it. Groudon is a Ground-type, which makes it susceptible to Water-, Ice-, and Grass-type Pokemon such as Vaporeon, Articuno, and Exeggutor.