https://www.gamespot.com/articles/pokemon-go-new-legendary-regirock-available-for-a-/1100-6461213/

Registeel may no longer be available in Pokemon Go, but a new Legendary has taken its place. From now until September 20, the third and final member of Pokemon Ruby and Sapphire's Legendary titan trio, Regirock, will appear as a Raid Boss at Gyms around the world.

Like the previous two titans, Regirock has high defensive stats, although that's offset a bit by its long list of weaknesses. Being a Rock-type, Regirock is particularly susceptible to Grass, Water, Ground, Fighting, and Steel attacks, so you'll want to leverage its many vulnerabilities to your advantage when taking one on.