My original theory was their could be a "nut" item in Gen 8 Ditto could hold, if it did and tried to breed with a Mew / Mewtwo you would get an egg which hatched Meltan.
Given we now know Meltan is a mythical Pokemon i doubt that will be the case, it's certainly going to be an interesting one however.
Screen capture of the first Meltan I captured on community day: