Well, technically shapeshifting is what Marco does. He just doesn't do the freaky skin shedding thing. Or the whole being a creature of evil thing.

He trotted closer, coming to see down in front of Dean's feet and look up at him. <you have fun with that then. Maybe sometime in the next decade or so, you'll actually finish it,> Marco scoffed. <Why the hell do you even need more rooms anyway?>