https://www.gamespot.com/articles/last-day-to-catch-pokemon-gos-current-legendary-ky/1100-6456690/

The Legendary Pokemon Rayquaza may have just arrived in Pokemon Go, but another Gen 3 Legendary is on its way out very soon. The cover monster from Pokemon Sapphire, Kyogre, will disappear from the mobile game today, February 14, making this your last opportunity to catch one if you still haven't.

Like most other Legendaries to appear in Pokemon Go thus far, Kyogre can only be encountered as a Raid Battle at Gyms. You'll first need to battle and defeat the Pokemon before you have an opportunity to capture it. As one of the most powerful Water-types in Pokemon Go, players will want to bring Electric- and Grass-type Pokemon such as Jolteon, Zapdos, and Exeggutor to gain the advantage over Kyogre.