Two new videos have now detailed more information about Meltan’s method of capture, behavior, and a possible forme!
The first time players send a Pokemon from Pokemon GO to Let’s Go Pikachu or Eevee, they can receive a special “Mystery Box” item in Pokemon GO. Upon opening the box in GO, Meltan will begin appearing in the game for capture, though only for about 30 minutes. The box will then close and will only reopen again after a set amount of time, at which point the player will need to send another Pokemon from GO to their own Let’s Go game or a friend’s. Players can also receive a Mystery Box if another friend sends a Pokemon to their Let’s Go games. (All of this is obviously a marketing method to get players and their friends to use Pokemon GO and Let’s Go as much as possible.)
A silhouette of a Pokemon resembling Meltan appeared at the end of the second video. Perhaps this is a forme of Meltan that becomes available once you’ve captured many of them? (Like a bunch of liquid metal coming together to form a large forme?) The videos do seem to promote the idea that a player should capture many of them, as the subject in the second video captures so many they begin to take over their house. And of course in GO you always capture many Pokemon of the same species, so Meltan was probably designed with that type of gameplay goal in mind. Meltan is of course a Mythical Pokemon — many Mythical Pokemon have had formes but none have ever had evolutions — so it doesn’t seem as likely the silhouette is an evolution. Of course this is all my personal speculation.