In Pokemon Go, Mew, Mewtwo, Ditto, Articuno, Zapdos, and Moltres have not been founded . How to get them and when they appear in the app are not revealed. There is no official announcement.
However, a video, showing those legendary Pokemon in gym battles, was posted on Youtube. Here are the list of legendary Pokemon, the video, and the screenshots.
List of Pokemon have not been founded in Pokemon Go
The Mew uses Hyper Beam, one of the charge moves, to defeat a Vaporen. The Cp of Mew is 2,134, it is lower than that of Vaporen, 2449. However, the Mew beats the Vaporen easily.
Mewtwo
The Mewtwo's CP is 1,704. It is not very high, but the Mewtwo defeat the defenders with one blow.