https://www.gamespot.com/articles/new-pokemon-go-legendary-now-available/1100-6460549/

The Legendary Pokemon Regice has now left Pokemon Go, but a new Legendary has taken its place. Registeel, the second of the three Legendary titans originally from Pokemon Ruby and Sapphire, has begun appearing around the world, and it'll remain in the game until August 21.

Like Regice before it, Registeel boasts high defensive stats, making it a formidable opponent. As with most other Legendaries, it can be encountered as a Raid Battle at Gyms. Players will first need to team up in-person with other trainers to battle Registeel; once it's been defeated, they'll earn a chance to capture it.