https://www.gamespot.com/articles/pokemon-go-adds-new-gen-3-legendary-regice/1100-6459964/

A new Legendary has begun appearing in Pokemon Go. Regice, one of the three Legendary titans originally from Pokemon Ruby and Sapphire, can now be found at Gyms around the world as a Raid Battle, where it will appear until July 19.

As usual, players will first have to team up with other Trainers and battle Regice before earning a chance to capture it. Like its name implies, Regice is an Ice-type, which means it's particularly vulnerable to Fire-, Rock-, Steel-, and Fighting-types. Despite its many weaknesses, it has high defensive stats, making it a formidable Pokemon to take down.