Most Pokemon GO players know that catching Ditto -- let alone the famous Legendary creatures -- is like a shot to the moon. Its spawn rate is almost close to zero, making it the rarest (for the lack of better term) specie to own.
As for the Legendary Pokemon GO creatures, almost all players know that they don't currently exist in the game. Nevertheless, as reported earlier, Redditors are believed to have found these along with Ditto in the game's source from the original 151 Pokemon.
However, prior to the aforementioned Pokemon GO source, analysts and data miners alike have already theorized the existence of both Mew and Mewtwo. They basically believe that these Pokemon will be introduced in the game as an Event Based. And true enough, these monsters have yet to be included in the current list of eggs, seemingly proving the aforesaid theory. If they eventually get added to the upcoming Buddy System, these creatures will prove useful in every gym battle.
As for the rumored Pokemon GO added km requirements, below are the breakdown for both Ditto and the Legendaries:
Ditto - km_buddy_distance is 3Articuno, Zapdos and Moltres - km_buddy_distance is 5Mew and Mewtwo - km_buddy_distance is 5
The above mentioned Pokemon GO added km requirements are basically ferreted out by data miners from the update's source file. Of course, these should be taken as a grain of salt, as Niantic has yet to confirm any of these. Nonetheless, this is really something to look forward to.