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.
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.
The Mewtwo's CP is 1,704. It is not very high, but the Mewtwo defeat the defenders with one blow.
The Articuno uses Twister, the dragon-type charge move, to beat Pinsir, even though Articuno do not learn Twister based on the revealed data.
The Zapdos figths against to Electabuzz, another electric-type Pokemon.
The Moltres uses Earthquake, the ground-type charge move. Moltres do not use the charge move according to the revealed data.
Not sure what move the Ditto use, but it is very strong. The Ditto defeats the Gyarados.
