Transferring Meltan from Pokémon Go to Let's Go Pikachu and Lets Go Eevee is the only way for the games to receive Meltan, as this Pokémon cannot be in encountered in those games. The Original Trainer for Pokémon Go was changed for privacy purposes.
Shiny Meltan (captured by @wo0ts) was temporarily added to the shiny pool during the Lunar New Year Event (2019).
During the Lunar New Year Event (2019) in Pokémon Go, shiny Meltan has a chance of appearing from using the Mystery Box, duration listed below.
This event also marked the first time a Shiny Meltan was made available to players.
The cool down between box usage were temporarily reduced to 3 days instead of the standard 7 days.
The slated period for the event was between Feb. 5, 2019, at 1 p.m. PST (GMT -8) to Mar. 4, 2019, at 11:59 p.m. PST (GMT -8).
Format Ver.1.0.6-3, Post Updated Date:20190105_0509