https://www.polygon.com/2017/8/31/16233010/pokemon-go-ex-raid-battle-field-test

Pokémon Go’s upcoming Exclusive Raid Battle system is on its way to the game. Niantic announced this morning that a revamped version of the new feature, EX Raid Battles, will enter field testing soon.

EX Raid Battles will work similarly to the current cooperative multiplayer mechanic, according to the developer. These were previously announced as Exclusive Raid Battles, which Niantic said would include limited-time, special Pokémon, including Mewtwo.