Legendary Pokemon Regirock will start to appear in Pokemon Go today.
As announced on the official Pokemon Go Twitter account, Regirock will come to raid battles today at 1:00PM Pacific Time and will remain in the game until September 20.
Regirock follows Registeel and Regice, who were added to Pokemon Go in June and July following the disappearance of Latios and Latias earlier this year. This means every Legendary Pokemon from generation three (which was originally introduced in Pokemon Ruby and Sapphire) has now been added to Pokemon Go.
Source: IGN.com Pokemon Go: Legendary Pokemon Regirock Added to Raid Battles