https://www.gamespot.com/articles/meltan-pokemon-gos-strange-new-creature-revealed-h/1100-6462019/

Many Pokemon fans were confounded recently when what appeared to be a strange, unidentified Pokemon began appearing in Pokemon Go right after September's Community Day event. The monster could only be found in the wild for 30 minutes (though it has since been sporadically sighted in the game), and when captured, it would reveal itself to be a Ditto or another Pokemon, furthering the mystery of whether or not it was a deliberate tease on Niantic's part. It turns out it was, and the unusual Pokemon now has a name. Here are all the details we have so far.

The Pokemon Company has confirmed that the strange, Ditto-like monster is a brand-new species of Pokemon called Meltan. In a new trailer the company shared, Professor Willow from Pokemon Go calls Professor Oak, the most prominent researcher from the main series, with news that Ditto masquerading as an unidentified Pokemon had been sighted recently. Oak says he recalls seeing the monster in ancient texts and that it's a Mythical Pokemon known as Meltan.