https://www.gamespot.com/articles/pokemon-go-adding-new-mythical-pokemon-but-it-will/1100-6461892/

Pokemon Go players will soon have their first chance to add another rare Pokemon to their collection, but this one may be harder to catch than most. The Mythical Deoxys from Pokemon Ruby and Sapphire versions is set to debut in the popular mobile game, but just as Mewtwo before it, it will only be available through EX Raids.

Unlike standard Raid Battles, EX Raids are an exclusive type of battle that you can only participate in if you received an invitation. In order to qualify for an invite, you will need to have recently won a Raid Battle at a "qualifying" Gym. If you do manage to get your hands on one, you'll now be able to extend the invitation to one of your Ultra or Best Friends.