Bodies have been found buried in peat bogs.
They are preserved by the natural conditions there, but rather like Egyptian mummies, or Ötzi the Ice Man, they are very shrivelled and wrinkled. I suggest, therefore, that people who are like latterday bogfolk are rather similar.
I would imagine it is describing the well-known bodies of prehistoric people who were buried in peat bogs (mostly in Ireland). The peat preserved their bodies beautifully and several have been found with their skin, hair, nails and internal organs intact.
They do look "shrivelled and drawn" - not surprising really.
I find them fascinating (in my studies in anthropology).
So we can say that the word "bogfolk" is "bog" + "folk", but is this combination just the invention of this author(Cormac McCarthy)? Is it so normal to coin new words with "folk"? Maybe "bog people" or "bogmen" would be more 'normal' expressions.
I've been searching a little ans there's lots of information aboput "bog people", "bog bodies" and only a few references of "bog-folk". But "bogfollk", written in one word, is practically non-existent. That's one of the reasons I couldn't find it initially.