Niantic have finally confirmed that the legendary Pokémon Articuno was 'erroneously' given out to a few people.
However, they've now made the move to remove them for their accounts, therefore it is not available any longer.
Got to be honest, if that was me and I was lucky enough to obtain one, I would be pretty pissed to get it taken off me without any prior warning and justification. They made an error but those few should have kept the pokémon. It wouldn't have give those tiny few an advantage imo.