Rayquaza isn't the only new Pokémon joining the game. Several Dragon and Flying-type Pokémon from the Hoenn region will also appear starting Friday. Afterwhich over 100 Hoenn region Pokémon will become available in the game, and to celebrate, Pokémon Go will make it so that starting today and until February 13, every Pokémon encountered in the wild will be from the Hoenn region.
However, just as new Pokémon from the Hoenn region come in, there will be those that leave the game as well. Kyogre will exit the game from Raid Battles on February 14, so if you haven't had a chance to try and capture the water-type legendary, best do so soon.
To help with your journey, the Pokémon Go in-game store will sell a new special box containing Raid passes, Incubators, and Star Pieces until February 23.