Marcargo, and by extension, Slugma, are not new additions to Pokemon GO by any means. What is a new addition, however, is the shiny form of Magcargo that was recently added into Pokemon GO, earlier during January in the Mountain of Power event.
Can Magcargo be Shiny in Pokemon Go?
Continuing the Heritage Saga Season, Mountain of Power began exactly on 7th January 2022, where players were incentivized to catch various Steel and Rock type Pokemon during the event in order to figure out more about the mysterious second door mechanism.
This event added a lot of new Pokemon into the augmented reality Pokemon catching game, as well as new shiny versions of Pokemon, one of them being shiny Magcargo, who had its shiny rates increased for the time being as well.
During the event, players had the chance to experience a guaranteed Slugma encounter when they caught five different Pokemon. This however, no longer exists as part of the game. But there is good news.
Slugma and Magcargo are still available to capture in Pokemon GO, as well as their shiny versions. They can be encountered in the wild, through field research events, raid battles, and so on. Their shiny rates are back to normal, however, so shiny hunting Magcargo is going to be the default process like it is for many other Pokemon.
Your best bet is to shiny hunt for Slugma as it is much more common than Magcargo, and then use 50 Candy to evolve your Shiny Slugma into a Shiny Magcargo. Magcargo is a strong Fire / Rock type to have in Pokemon GO, so shiny hunting it will be well worth your time.