https://www.gamespot.com/articles/pokemon-go-last-chance-to-catch-legendary-pokemon-/1100-6460427/

The Legendary Pokemon Regice has been a fixture of Raid Battles since it debuted in Pokemon Go last month, but it won't be available for much longer. The Legendary Titan is only scheduled to appear until this Thursday, July 19, making this your last opportunity to add one to your collection before it leaves the game.

As with other Legendary Pokemon, players will first need to team up with other Trainers in-person at Gyms to battle Regice, and only after the Legendary has been defeated will they earn a chance to capture it. Just as its name implies, Regice is an Ice-type, which makes it especially vulnerable to Fire, Rock, Steel, and Fighting Pokemon, such as Blaziken, Machamp, and Tyranitar.