Calyrex's steed doesn't have two forms -- it's two different Pokemon. The Ghost type is Spectrier, the Ice type is Glastrier.
Calyrex is said to have ruled Galar in ancient times. Calyrex was referred to as the King of Bountiful Harvests, for the lush bounties of vegetation it brought upon Galar's land. Over time, the traditions of the people that worshipped Calyrex faded, and Calyrex was all but forgotten.
Glastrier serves as one of Calyrex's steeds. Before Calyrex tamed it in ancient times, it attacked the citizens of Galar in an effort to steal their resources and generally cause havoc. Once Calyrex grew weak from loss of faith, it abandoned its master, and ran free once more. Over he course of the Crown Tundra's story, it comes back to serve Calyrex once more.
Spectrier is much the same as Glastrier. It, however, has the ability to steal the life forces of people, but it too enjoyed terrorizing people before Calyrex brought it under control.
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