Pokemon Go unveiled the first promised legendary Pokemon at the Pokemon Go Fest in Chicago today, and it's Lugia. Despite, or perhaps because of, the problems reported at the festival, players at the Pokemon Go Fest will get the legendary Lugia in their accounts.
Lugia will start appearing in raids for players around the world in 48 hours, but players in Chicago's Pokemon Go fest will get Lugia without having to participate in the raid. Niantic previously announced that legendary Pokemon raids will be a part of the festival, but due to the technical issues affecting the festival, Niantic is instead giving away Lugia to attendees.
Also, since Team Mystic won the mystery challenge as part of the festivities, Articuno will be the second legendary Pokemon. Team Mystic players apparently caught the most Pokemon as part of the global Pokemon Go event, and as a result, their mascot legendary Pokemon, Articuno, will be the next legendary release. However, there's release date announced yet.