Please note that the following is my own opinion, and is not supported by anything Tolkien explicitly wrote or said.

If you mean the rock hurling giants from The Hobbit, the answer, is, as far as I know, Tolkien never speaks of them elsewhere so we don't know. I assume they are spirits of the mountains. Remember when the Fellowship was snowed in on Caradras? That was the Genus Loci of the mountain trying to stop the company, not because it was in league with Sauron, but for reasons of its own. There are probably many more forces like that who have been there since the beginning, who hate and despise everything that moves on two feet. You could liken them to evil Bombadils. The rock hurling giants were in my akin to these. I assume that originally they must be soome sort of Maia.