Goomy congeal below the neck when exposed to cold enough temperatures during the process of evolution, and it is this phenomenon which lies behind the so-called "Hisuian" variant of Sliggoo. The shell variant of Sliggoo were first described in Hisui, where they seem to have been so common as to supplant the normal variety of the species, but one need only go a hundred miles or so north of Unova to find specimens of this pokemon mixed in with their soft tissue counterparts. They are the normal form of Sliggoo not only in Hisui, but wherever temperate forest gives way to taiga.
As dragons (with the associated long dragon life-cycles) Goomy struggle to survive so far north, and a high proportion of those found outside Hisui were born in the temperate zone but pushed out of their desired range. Hisuian Sliggoo (and Goodra) are perhaps the best adapted of all the mortal dragons to cold climates, but this is damning with faint praise; it is a highly defensive species which was never feared by the hunters of the circumpolar areas the way Shelgon or Vibrava might be further south. The Snorunt they prey upon rarely recognize Sliggoo as a threat, and they are rare enough that pokemon must learn through experience not to feed on them, but also conflict-averse enough that one typically comes away with nothing worse than the frustration of wasted time.
This is not, however, to say that Hisuian Sliggoo are harmless. There is a mighty beast which lurks within that gooey shell, one whose rampages go unrecorded only because there is so little in such a remote land to destroy – except people, one supposes, and it does not leave survivors. Only the fae can safely make light of dragons.