Katy, it could be a Marans egg...sometimes they lay one colored like that. If they lay a lot and close together (I've got a hen that will sometimes lay two eggs in 24 hrs), they kind of run out of paint in the chute, so to speak, seeing as the dark egg color is really a coating on a lighter color egg. I've occasionally gotten an egg that was a regular brown color w/just a spattering of the typical dark Marans "paint" on it, like the speckles on your ginormous egg!!