The Roc is a gigantic bird, "almost too big to be believed", that makes large animals such as horses, elephants, and small whales its prey. I love the idea of ordinary creatures blown up to extraordinary sizes, and whilst there isn't anything especially glamorous about the Roc, it's nice in a simple sort of way.
There's this picture of one I find really funny where it's holding a hapless whale in its big claws, so I thought I'd take that idea of this gigantic predator and make it a little more dynamic.
