The Legendary Pokémon Groudon has begun appearing around the world via Raid Battles in Pokémon GO. It can be found and caught until January 15, 2018. The official announcement from the Pokémon GO development team at Niantic is as follows:
Groudon, the Ground-type Legendary Pokémon originally discovered in the Hoenn region, is now appearing in Raid Battles around the world! Keep an eye out for Groudon at Gyms near you, and you may be one of the first to catch this Legendary Pokémon. But don’t delay—Groudon will only be available until 1:00 p.m. PST on January 15, 2018.
Good luck, Trainers!
—The Pokémon GO team
Source: Official Pokémon GO blog