UPDATE 22/9/18: Yesterday's leaked new Pokémon design has stunned fans by suddenly appearing in the game. What appeared to be a slip-up now looks like a purposeful release, as spawns of the unnamed creature began appearing overnight.
Today is the game's monthly Community Day event, which takes place one day each month over a three hour window. This morning, when the game's Asia/Pacific Community Day hours concluded, spawns of the new Pokémon suddenly flooded the game for 30 minutes. Here in the UK, at the conclusion of the European Community Day hours, the same thing happened.
But any attempt to click on the Pokémon and catch it results in the creature simply turning into a Ditto - the Pokémon which mimics the look of others. While Pokémon Go players can't catch it yet, then, its appearance as a Ditto seems to confirm it is legitimately a new Pokémon design.
Is it the new Pokémon to be released in Let's Go? Is it an odd form of the shape-shifting Kecleon, which is also due a Pokémon Go release? Was it always planned for release in this way, before the leak? When will players be able to capture it properly?
One thing is for sure - Pokémon Go has never had a mystery like this. In the meantime, here's some theories on what the Nut Pokémon could be.
ORIGINAL STORY 21/9/18: A brand new Pokémon may have been accidentally revealed, fans think, after a bizarre creature design just appeared in behind-the-scenes Pokémon Go data.
The design in question looks like a Ditto - the gelatinous shape-shifting creature from the series' original 151 designs - with a metal nut on its head.
Ditto Cog, InCognito, or whatever you want to call it, popped up last night in files found by serial Pokémon Go leaker Chrales. It looks like the design was added while developer Niantic was uploading data for Kecleon - one of the few remaining unreleased Pokémon from Gen 3.
How could a new Pokémon suddenly appear? Well, this year's Pokémon Let's Go: Eevee and Pikachu will include one brand new Pokémon species. Let's Go and Go will link up to enable the transfer of Kanto Pokémon, and with the game's release now less than two months away, it would be fair to assume Niantic has been working with Let's Go code to enable this to occur.
The other school of thought is this design is just some kind of elaborate placeholder - although this would be unprecedented. Pokémon Go already has a placeholder icon for unfinished Pokémon. Additionally, this design has been crafted in 3D with both regular and Shiny versions - adding to the feeling this is a final design.
Here it is:
Despite being labelled as Kecleon, this design holds the number 891 - well ahead of the current tally of Pokémon up to the end of Sun and Moon's Gen 7.
We've contacted Niantic for comment.