Producer Shigeru Ohmori and director Kazumasa Iwao announced that the players can catch mewtwo in the upcoming Pokémon Ultra Sun and Ultra Moon.
If one reads between the lines presumably there will be other legendary Pokémon that the player will be able to catch in this world, though that information is just being hinted at. In the same interview Ohmori and Iwao highlighted that catching and training Pokémon will be easier in Ultra Sun and Ultra Moon.
The ability to catch legendary Pokémon other than MewTwo and those native to the Alola region is an important selling point for the game. The chance for players to add previous legendary Pokémon that are not yet in their collection is an significant way of enticing the community to buy Pokémon Ultra Sun and Ultra Moon. By expressing that this game will make it easier to catch and train Pokémon, Nintendo seems to be trying to secure the demographic of the Pokémon collectors.