A Niantic representative has revealed to Geek.com that some people in the US got Articuno by mistake and that it would be rectified by being removed from their the Legendary from their accounts. You can read that statement below.
“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.”
The only thing that still isn’t clear is whether the representative means that it mistakenly got put their account from Niantic or whether the couple used some workaround to have it appear in their game.