A legendary raid for Pokemon Go has started in Japan, and with it came the first appearance of Mewtwo in the mobile game. Japanese users assembled in mass to Yokohama stadium for a timed 10-hour raid in which they had a chance to capture the legendary Pokemon.
— Nicholas Oyzon (@trnrtips) August 14, 2017
People are reporting a 100% catch rate for taking part in the event, so it is a good thing that you will catch it if you ever get to see it. it will appear sometime soon in America and you have a good chance of catching it.