Shadow Mewtwo Pokemon GO was apparently leaked as dataminers discovered the Pokemon in the game's files.
With the addition of Mega Evolution to Pokemon GO, it was already clear that Mewtwo was about to become more powerful. Based on info from a recent datamine, the extent of the increase might be greater than previously thought. PokeMiners (@poke_miners) discovered newly added assets for shadow and purified forms of Mewtwo in the Pokemon GO GameMaster.
Shadow Mewtwo Pokemon GO: Dataminers Discover Possible Addition
Mewtwo is already strong enough on its own, but the power boost afforded by a shadow form has the potential to rocket it to the top of the leader board in terms of DPS.
Additionally with the upcoming Mega Evolution update and a theoretical shadow form Mega Mewtwo Y in the cards, we could be looking at a potentially game breaking shift in power. It's possible that shadow form Pokemon might be prevented from mega evolving much like how they cannot forget frustration until purified or mega evolution.
Perhaps Shadow form will increase the damage boost against the Shadow Pokemon, but we'll just have to wait and see when Shadow Mewtwo is officially released.