Check this announcement by Niantic in which a new feature called Exclusive Raid Battles is revealed along with the expansion of the time Legendary Birds will stay with us:
Earlier today, Trainers battled against the Legendary Pokémon Mewtwo at the Pokémon GO Stadium event in Yokohama, Japan. Thousands of Trainers in attendance were able to successfully catch Mewtwo and add it to their Pokédexes. In the coming weeks, you, too, will have the opportunity to battle and catch Mewtwo with the new Exclusive Raid Battle feature.
Exclusive Raid Battles are similar to existing Raid Battles, with a few notable differences. Exclusive raids will periodically appear at Gyms around the world; however, unlike existing raids, Trainers will be invited to join an Exclusive Raid Battle. To receive an invitation to participate in an Exclusive Raid Battle, Trainers must have successfully completed a raid recently, by defeating the Raid Boss, at the Gym where the Exclusive Raid Battle will be taking place. The invitations will include advance warning of when the Exclusive Raid will take place, giving them ample time to coordinate with other Trainers before taking on the powerful Raid Boss.
Mewtwo is the first Pokémon that Trainers will be able to face in Exclusive Raid Battles, but other powerful Pokémon may also start to hatch from the Raid Eggs found at Exclusive Raid Battles over the next several weeks. Make sure you’re prepared to battle Mewtwo by powering up your Pokémon and battling in raids at Gyms near you!
Legendary Pokémon Take Flight
Over the past three weeks, thousands of Legendary Pokémon have been defeated in Raid Battles around the world. In celebration of this achievement and to give Trainers another chance to catch Articuno, Zapdos, Moltres, and Lugia, all four of these Legendary Pokémon can be battled in Raid Battles from August 14 through August 31. Be on the lookout for these Legendary Pokémon at a raid near you!
—The Pokémon GO team”
Legendary Birds will be here until the end of August and as for Mewtwo, it will be invite-only. Invitation will be handed ONLY if Mewtwo appears on a gym in which you have recently played a raid battle and won. In fact, invitation will be arriving earlier in order to organize with the rest players that have received one.
Happy Hunting…see you out there!!