"Pokemon Go" players have been waiting since the game's initial release for the inclusion of the first generation's Legendary Pokemon. Articuno, Zapdos, Moltres, Mew, and Mewtwo—as well as Ditto, who isn't really legendary, but hasn't been included yet—have yet to be included in the game, and all we know is that there are plans to put them in.
What we can gather from poking around "Pokemon Go"'s source code is that although they're not in any recent updates yet. They should be in the game pretty soon—that, or they're already in, and we just haven't found them yet. The recent "Pokemon Go" update added the Buddy System to the game, where players can assign one of the Pokemon in their inventory to be their buddy and walk around with them to get Candy. The code provides for the distance requirements needed for a legendary buddy to generate some Candy.
Otaku Kart has published these requirements for "Pokemon Go." For Ditto, players will need to walk 3 km in order to generate Candy, while for Articuno, Zapdos, Moltres, Mew, and Mewtwo, they'll need to walk 5 km. Other speculation about the legendary Pokemon also include the fact that being able to use them as Buddies (and the mere act of being able to gather Candy for them) means that they'll most likely also be useable for Gym battles.
We just don't know when "Pokemon Go" developer Niantic is finally going to make them available. For all we know, they could already be out there hiding somewhere, but with all the "Pokemon Go" players in the world, they would have found one if they were already out in the wild. Niantic is going to need to implement a lot of major changes to the game to keep players interested in it, as it has recently been reported that the game's player base has been falling. We'll bring you more updates on "Pokemon Go" as soon as we get them.