UPDATE 03/08/16 6.54pm: Niantic are removing legendary Pokémon from people's accounts.
A studio representative sent the following statement about the matter to IGN:
"We recently noticed that a few Legendary Pokemon 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 Pokemon from the Trainers' accounts."
UPDATE 3/8/16 2.30pm: Articuno's owner has shown off the legendary Pokémon in an extended Twitch stream, in a bid to prove her story is legit.
Yesterday, Kait Covey from Ohio claimed she had the world's first legendary bird Pokémon. Her somewhat dubious explanation? She claimed Articuno had been gifted to her account by the game's developer, Niantic, after a bug made her lose one of her own.
Still, screenshots and video from other players in her area who had seen the Pokémon sat atop gyms apparently backed up the story.
Now, Covey and a friend have taken to Twitch to show off Articuno in full.
Let's be clear - it is difficult to believe Pokémon Go developer Niantic would randomly gift the world's first Articuno in this way.
But this only makes the truth behind the Articuno mystery all the more interesting. The possibility remains that the couple genuinely believe they have a real Articuno from Niantic but are being spoofed by a third party. It could all be real, just as they've explained it. Or, simply, they're lying. Whatever you think, the video below is convincing - without a doubt.
If you click through to watch on Twitch, you can see the Coveys responding to archived chat messages and attempting to prove what they can. The couple show the game running on Wi-Fi, on data, on a different phone, then two phones logged into the same account. They show the email Kait Covey received from Niantic - although, oddly, Articuno is misspelt here as Articundo.
We've contacted Niantic in case it wants to corroborate or quash the story.
For now though, you can make your own mind up by watching the stream below:
ORIGINAL STORY 2/8/16 11.45am There's been a rare sighting in Pokémon Go. No, not a new community update from Niantic - something far more unusual: the legendary Pokémon Articuno.
Articuno is one of few critters you can't currently catch. Fans have snooped into Pokémon Go's files and seen Articuno in there, waiting for release. But, to date, it has remained unobtainable.
Until now! Maybe.
Multiple Reddit users have spotted an Articuno roosting atop gyms in Ohio, and provided screenshots and video to back this up.
That people are seeing an Articuno in their games is not in doubt. It's there, but what it really means is another thing entirely.
Articuno's owner, kaitcovey, posted about her prize on her local Pokémon Go Facebook group. Her story is that she received the Pokémon as part of an apology from developer Niantic, after contacting them via email about an issue.
It sounds possible, but unlikely - there has been no suggestion that legendaries would be given away in this manner. Rather, the expectation is that they are being saved for large public events at key locations.
Also, considering the rather predictable anger among fans about the way the ultra-rare Pokémon have apparently first appeared, it would be another awful PR move.
There are other explanations, however.
One possibility is that the Pokémon's data has been modified so that the game identifies it as Articuno. It would require the editing of the creature's data via an external tool - which is not beyond the realms of fantasy. It would also mean another headache for Niantic to fix if the exploit became widespread.
Another possibility is a glitch in the game which has accidentally released an Articuno or has somehow glitched another Pokémon into Articuno once caught. There was a report today of a visual bug which caused a Caterpie to reveal itself as a Charizard once captured.
The issue is confused by others jumping on the bandwagon with more obvious fakes.
Whatever the truth, it's currently another point of contention for fans of the game.