Many people thought this addition to the game was the result of a hack or glitch. However, today, Niantic revoked several legendary Pokemon from from player accounts. According to a statement from Niantic to IGN,
As of right now, the developer has not officially released any legendary Pokemon to the game. They are more preoccupied with fixing the existing problems in Pokemon GO: such as unstable servers, lack of a working tracking system, as well as other issues.
While these Pokemon can still be obtained, it can only be done with a glitch or hacking the game. There is no word from the developers when we will see legitimate legendary Pokemon, but let’s hope the initial glitches are fixed first so we can start enjoying the new features.