https://www.gamespot.com/articles/pokemon-gos-mewtwo-now-available-in-raid-battles-f/1100-6461935/

After their brief return as part of the Ultra Bonus Event, Articuno, Zapdos, and Moltres have once again left Pokemon Go, but a new Legendary has taken their place. Mewtwo, the powerful Psychic-type from Pokemon Red and Blue, is now available in standard Raid Battles around the world, giving most players their first chance to capture it.

This isn't the first time Mewtwo has appeared in Pokemon Go; the Legendary Psychic-type has been available for several months now, but it could previously only be found in EX Raids--an exclusive type of Raid Battle that players could only participate in if they receive an invitation. This restriction meant that most Pokemon Go users didn't have a chance to fight one, let alone capture it, so its appearance in standard Raid Battles is a welcome sight.