You can think of The Rook as eating the imagination of the Giants and thereby destroying bits of the world. It is like the personification of fear or self-doubt – the anti-creativity which threatens the world of the Giants' minds.
That rook looks a lot like local Austin bird called a grackle. They aren't harbingers of doom so much as delivers of bird poop. Lots of it. Especially when you are running across campus to class, through the gackle gauntlet near Gregory Gym.