Many of us started playing Pokemon in what's called the "Main Series"; the games that have been traditionally released on Nintendo's handheld game systems since the Gameboy. These games are where new Pokemon tend to appear first, with each new chapter in this series being referred to a "Generation", with each generation typically being connected to a "Region", where-in a given generation begins; Kanto, Kalos, Sinnoh, ect.
While it is not possible to move Pokemon from these main series games into Pokemon Go currently, it is possible to move Pokemon from Pokemon Go to a few main series games! To help make this process easier, we've compiled resources to help explain the process and just what it will entail for each given way to make this transfer.
The Meltan Box
As of right now, using these methods are the only way to obtain the mythical Pokemon Meltan, or its evolved forme, Melmetal.
After transferring a Pokemon via these methods, the Meltan Box becomes usable. When activated, it acts as a one-hour incense that will attract only Meltan. After this period, the Meltan Box may be activated by transferring another Pokemon five days after the last activation of the box.
Note that Niantic occasionally releases events that include Shiny Meltan spawning from the Meltan Box.