Niantic announced that from December 15th, 2017 until January 15th, 2018, Groudon will be appearing in Raid Battles around the world in Pokémon GO.

Groudon Appearing in Raid Battles around the World!

Trainers, 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!

Source: https://pokemongo.nianticlabs.com/en/post/legendary-groudon