Legendary Pokémon are finally in Pokémon Go, ready to be captured. They include the Legendary Birds Lugia, Articuno and Moltres, as well as the powerful Mewtwo.
After being unlocked at Pokémon Go Fest in Chicago, the two avifauna Articuno and Lugia can be found at gyms worldwide. Moltres appeared in the wild soon after. To capture them, you'll need to beat them in Raid Battles. You won't be strong enough on your own, so bring friends or meet other trainers.
Mewtwo is a little tougher. It's part of an "exclusive Raid Battle," which Niantic detailed here. Essentially, to be invited, you will have to have beat a public Raid Battle at a gym before, and you'll get an invitation with some advance warning if Mewtwo will happen to show up at that gym again. It's complicated, and will likely only attract die-hard fans.
The Legendary Raid Battles will be signified by special eggs sitting on top of gyms, indicating one of the two birds ready to hatch. Be sure to bring lots of Razz Berries to increase your chances: rumor has it that the successful capture rate sits at 2 percent.
Developer Niantic suggests that Zapdos and Moltres, the other two legendary birds from the original set of games, will be available soon. No word on Ho-Oh, which debuted with Lugia in Pokémon Gold and Silver, as well as the legendary dogs, Entei and Suicune, as well as Mew, which have been teased before.