Recently, there were several reports from Pokemon Go players that they have fought a legendary Pokemon in Pokemon Go game. For an example, you can find even a video which shows Articuno in the game. However, as far we know, it should not be possible to catch this Pokemon at the moment.
This statement was confirmed by Niantic Studio. They claim that these legendary Pokemons which are present in the game are result of the error. Of course, you will be able to catch them, but not now. Legendary Pokemons will be available in Pokemon Go, but they will be unique and available during special events.
Therefore, Niantic Studio has removed all legendary Nintendo monsters which appeared on accounts of Pokemon Go trainers.
The official information about this is very clear. It should not be possible to get a Legendary Pokemon at this moment of game, so Niantic Studio had to remove them in order to keep the balance.
We recently noticed that a few Legendary Pokémon got into a few accounts when they shouldn’t have. To preserve the game’s integrity and as a measure of fairness, we have rectified the situation and revoked the legendary Pokémon from the Trainers’ accounts.
There is no any official information about when legendary Pokemons will be available and how it will be possible to catch them. It is quite probably that you will have to have a Master Ball in order to accomplish this hard task. This is the best Poke Ball which is present in the code of Pokemon Go game, but currently it is not possible to get this.