Skellar are a subdivision of the Stillborn people. They appear either as grey skinned hairless half-zombie like humanoids or those who have body colorations and outfits that give them the appearance of being a skeleton. The floating head people with the detached bodies, for example, are not Skellars but are Stillborn alongside the Skellars. Deathboy and Skelebarbra are notable Skellars.
Amon is using some of their genetic make to influence the outcome of his Goetians.