Stone-Giants

[Mhoram]

I recently remembered the 'stone-giants' from the Hobbit and I have never figured out what they were. Here are some quotes:

When he peeped out in the lightning-flashes, he saw that across the valley the stone-giants were out and were hurling rocks at one another for a game, and catching them, and tossing them down into the darkness where they smashed among the trees far below, or splintered into little bits with a bang.

They could hear the giants guffawing and shouting all over the mountainsides.

"If we don't get blown off or drowned, or struck by lightning, we shall be picked up by some giant and kicked sky-high for a football."

"I must see if I can't find a more or less decent giant to block it up again," said Gandalf

These Giants seem to clearly be living breathing things, and not an illusion to a mountain or a storm. They can't be trolls because they would have been called such. No where else in the Legendarium have I heard tale of them, anyone have any thoughts?