One way to get mewtwo is defeat the game and then go on to the cave and catch him. The other way (you must have beaten the game first) go to the where giovanni is the gym leader and talk to the guy that asks if you want him to show you how to catch pokemon select yes and watch when he is done talking fly to cinnabar island and go the east coast and surf up along the coast keep going up and down and you will find after a while mewtwo and you can catch him. (Mewtwo will be around level 155) There will also be other pokemon like snorlax and all the rest in the version.