Meltan

At the end of Chikorita Community Day, this strange pokemon that appears to have a body that looks like a white Ditto and a head that looks like a brass nut.

Ditto
Whenever ??? is caught it always turns into a Ditto. People also noticed that sometimes, it turned into Chikorita. In some places, Kecleon was appearing but it also turned into Ditto.

Pokedex
When a person named Chrales looked at Pokemon Go's code, he discovered that this pokemon is labeled as Kecleon but #891 in the pokedex which leads to the fact that it is an accidental leak to a new batch of Pokemon from Gen 8 because there are currently 807 registered pokemon.