https://www.gamespot.com/articles/a-weird-pokemon-appeared-in-pokemon-go-and-no-one-/1100-6461982/

Pokemon Go's latest Community Day took place this past weekend, and by all accounts, it was a fairly standard event. However, something rather unusual happened right after the Community Day ended: a strange, never-before-seen Pokemon briefly appeared in the wild. [Update: The Pokemon Company has confirmed the unusual new monster, and its name is Meltan.]

In each region, an unidentified Pokemon began swarming immediately after the event concluded. No one knows what kind of monster it is; it was first discovered in the game's files shortly before the Community Day by dataminer Chrales, but it was listed with the name Kecleon--a chameleon Pokemon first introduced in the Gen 3 games Ruby and Sapphire.