Shiny Meltan may appear again when opening a Mystery Box in Pokemon Go from April 24, 2019 at 1:00 pm PDT until May 5, 2019 at 1:00 pm PDT. During the event, and after the event ends, you will be able to open a Mystery Box every three days.
The Mystery Box works just like an incense for Meltan; you open it and Meltan will appear around you for 30 minutes. To receive (or re-activate) a Mystery Box you need to connect your cellphone to a Nintendo Switch and send a Pokemon from Pokemon Go to Pokemon Let’s Go Pikachu or Eevee. Meltan evolves into Melmetal using 400 candies, so remember to use Pinap Berries with every throw!
Meltan and Melmetal
Shiny Meltan and Melmetal