Source: Niantic
The Regi Trio: Regirock, Regice, and Registeel are coming back to Pokémon Go for one weekend each in December 2020. These Legendary Raids will appear alongside Kyurem Raids, so you might have to do a little legwork finding them. However, if you do, all three have the potential to be Shiny.
Who is Regirock and the Legendary Titans?
Source: The Pokémon Company
The Legendary Titans are six golem-like Pokémon with a single type each. The first three, Regirock, Regice, and Registeel were originally encountered in the Hoenn region of Gen III. The fourth, Regigigas, didn't show up until the Sinnoh region of Gen IV, and two more, Regielecki and Regidrago were introduced in the Crown Tundra DLC for Pokémon Sword and Pokémon Shield. Only the original three will be coming back to Pokémon Go this time, but with the introduction of Mega Pokémon and lots of new Shadow Pokémon, there are a number of winning strategies for Trainers who can coordinate these Raids.