https://www.gamespot.com/articles/next-pokemon-go-legendary-now-available-for-a-limi/1100-6456089/

The Legendary Pokemon Kyogre is now available in Pokemon Go. Beginning today, players can encounter the powerful Pokemon at Gyms as a Raid Battle, where it can be found for the next several weeks.

As with other Legendaries, you'll need to team up with other players to battle Kyogre; only after defeating the Legendary will you have a chance to capture it. Kyogre is a powerful Water-type Pokemon, so players will want to bring Electric- and Grass-types such as Jolteon, Zapdos, and Exeggutor to gain the advantage over it.