Niantic has announced that we can expect to see Mewtwo at exclusive raid battles in Pokémon Go. These exclusive raid battles work as follows. The trainer needs to have participated in a recent raid battle at the gym in question to receive an invite (and advance warning) of the upcoming exclusive raid battle. So far, Mewtwo is the only confirmed exclusive raid battle, but in the future others will follow. There will also be raid eggs at these exclusive raid battle, presumably allowing you to hatch more exclusive Pokémon not available in eggs up to this point.
Furthermore, raids with the legendary birds, Articuno, Lugia, Zapdos and Moltres will open up until August 31, allowing you extra time to catch these for yourself.
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