the official Pokemon GO website published an interesting post, highlighting how Mewtwo is getting released worldwide – via Exclusive Raid Battles!
Exclusive Raid Battles are similar to normal Raid Battles, with a few differences:
Mewtwo is the first Pokemon that will be available in Exclusive Raid Battles, but more Pokemon are expected soon. These Pokemon will be available via special Raid Eggs found at Exclusive Raid Battles.
We’ll be updating our Legendary Raid Egg Boss List (Dark Egg List) listing and appropriate Hub pages soon. Stay tuned!
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.