The first answer was inaccurate, so I’ll be answering this myself.
Ok so, like Felix pointed out in his comment, Minior and Castform how have shinies for their different forms. Minior forms used to all just have a black shiny in core form and no shiny in meteor form, but now shiny Minior has alternate shinies for its core form. For example, it looks like there are both purple and green shiny Minior core forms, and presumably this is the case for other color Minior. Their shinies are most likely based off of what their core form would usually look like. Note that their meteor form still doesn’t have a shiny.
As for Castform, regular Castform always had a shiny, but its weather forms just looked normal. Now there are shiny colorations for each form that Castform can take.
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Also, yes, Magearna now has a shiny.
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Combusken's shiny was changed to be more noticeable with the release of the Crown Tundra.
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Hope this helps! :)
