[Mattius]

I always thought of their mention as metaphorical. When the Dwarves, Gandalf and Bilbo are on the mountain path in the storm Thorin (I think) mentions that he doesn't want to be picked up by a giant and kicked like a football. Perhaps they were once real creatures and by the time of the Hobbit were mythological and used in reference to such situations when there were avalanches in the mountains.