Cast your mind back to the official trailer for Pokémon GO. It sold the concept well enough, showed off the forthcoming Pokémon GO Plus accessory, teased the (still unavailable) trading element and culminated in a "raid" against the one and only Mewtwo, in which many players came together to bring down the elusive pocket monster.
If Mewtwo featured so prominently in the trailer, why hasn't anyone found him - or any of the other famous legendaries, such as Mew, Articuno, Zapdos and Moltres - in Pokémon GO yet? The lack of any confirmed sightings in the wild might lead you to assume that these iconic Pokémon aren't going to feature in the game, but dataminers have found references to them already, so that theory is out (bonus: Ditto is also in the game, it would seem):
While some are holding out hope that the legendary Pokémon can be found once your trainer level reaches a certain point, the trailer - with its epic multiplayer Mewtwo attack - hints that the character won't appear until a scheduled time. This is something that developer Niantic did a lot with its previous mobile game, Ingress. Dubbed "Anomalies" in that particular outing, these events would take place in certain locations over the course of a few weeks.
It's also possible that some of the legendaries - Zapdos, Articuno and Moltres - are tied to the team you pledge your allegiance to once you reach level 5. Perhaps you can only get one of these Pokémon, depending on which team you've joined.
Given that Pokémon GO's future seems to be tied in with the concept of sponsored locations, we probably shouldn't rule out the possibility of businesses paying top dollar to host a certain Pokémon which is exclusive to that location. If that sounds outlandish, it's worth remembering that this has already happened with the mainline Pokémon games - Mew was recently offered as a free download for Pokémon X & Y and Pokémon Omega Ruby & Alpha Sapphire at select retail stores.