Attack of the Stone Giants

Attack of the Stone-Giants was part of the Quest of Erebor. 
Stone-giants are described as hurling rocks at one another as in a game, during a violent thunderstorm. Their physical form was not described, nor was much else about them.
Gandalf used to tell stories to Bilbo when he was young at parties about dragons, goblins, giants, and rescue of princesses, and the luck of widow's sons. During the riddle game Bilbo thought of all the giants and Ogres he had heard of in tales.
At one point Bilbo saw a rock in a stream, and wondered if it had been cast miles into the plain by some giant among giants. He also compared being struck like the crash of battering-rams made of forest oaks and swung by giants.
Gandalf later remarked that he would like to find 'a more or less decent giant' to block several of the orc caves.
These stone giants were depicted as fighting each other as well as throwing rocks. They appeared as hewn from stone, and as the size of mountains - indistinguishable from a cliff-face when still. This is somewhat reminiscent of the more tree-like design of the Ents in the previous trilogy, with the giants appearing more inspired by nature than by humanoid appearance.