The Roc is a gigantic bird from Arabian mythology and folklore. It naturally got picked up by fantasy games, appearing in the first monster manual of every edition of both D&D and Pathfinder. The Roc is described as being large enough to block out out the sun, and to be able to seize adult elephants in one talon. Marco Polo described their flight feathers as so large it took a grown man 12 paces to walk its length. Needless to say, due to balance issues, Rocs in D&D aren't usually that big, though they do tend to be at least close to the Gargantuan size range.
In the World Axis cosmology, rocs have some unexplained connection to the Elemental Chaos, making them the most mundane of an entire family of giant bird creatures, like the storm-aspected Thunderbirds, sort of the avian counterpart to Hill Giants.