Niantic is going all out for Pokémon Go's first anniversary. In addition to holding the Pokémon Go Fest in Chicago and tie-in events all over the globe, it's also finally introducing Legendaries into the mobile game. You'll be able to find Legendary eggs at gyms, but since the Pokémon type is incredibly rare and powerful, you likely won't find one at every turn. Niantic is also using Legendaries as raid bosses, and if you and your team defeat one, you'll get the chance to catch one of your own. You can't use them to defend gyms, but hey, you'll need to catch them if you want to catch 'em all.
The first Legendary might make an appearance at the Pokémon Go Fest at Grant Park on July 22nd. If the attendees and participants elsewhere catch enough monsters, Niantic will unleash the first Legendary Raid Boss. And if the attendees in Chicago can beat it, the boss will start popping up in Raid Battles all over the world after the festival.