Mewtwo is a Psychic Type Legendary Pokemon that is very powerful and often use due to it's strength in the game. It can be found anywhere where you can encounter wild Pokemon and has a 1/1000 chance of appearing, however you can make it easier to find Mewtwo by buying the "Kanto Legendaries" gamepass which will make Mewtwo a 1/100 chance of encountering. You can also use the Legendary booster in the menu tab to make it a little bit easier as well.