Final Update?: Niantic confirms several legendaries were accidentally given to users and have since been revoked http://www.ign.com/articles/2016/08/03/pokemon-go-dev-revokes-legendary-pokemon-from-trainer-accounts-following-articuno-appearance
Update: The owner of the articuno did a livestream trying to prove the legitimacy https://www.twitch.tv/endersgw/v/81373999
As far as I’m aware the only way to fake these pokemon is to set up a local server on your own wifi effectively creating a private server that does not sync to niantic’s servers. The livestream shows the articuno still there after switching to mobile data. Another possibility is running the server off the android device itself somehow, but they also showed a fresh install of pokemon go on an ios device that appears to be unjailbroken. I’m not sure where people are getting the idea that you can modify the game files to reskin a pokemon but I wouldn’t be surprised if that was possible. As for the comments saying it looks like a pidgeot, the code for the legendaries are in the game and they can be seen using the MITM local server. This is the real model for Articuno.
A friend showed me a facebook link with some poor quality pictures of an articuno in a gym battle, so I figured out where the gym was but it had been taken down. A few minutes after the person who owns the articuno took another gym a little bit away and I managed to find it using nox.
More information can be found here https://www.reddit.com/r/pokemongo/comments/4vql8r/multiple_screenshots_by_different_people_have/
I did not fake the articuno, the articuno was there and is real. Feel free to disagree, not much I can do to convince anyone. The only thing faked in this video is my gps location.