Everyone likely knows that Nightmare's skull is 'two' parts, in a sense. His jaws are connected by a thin piece of fleshy material that keep it from falling apart. This is an example of... such a thing and just how stretchy said skin is.
Nightmare's meant for creeping things out, and giving other deer Nightmare's. I'm sure he can pull that off if he did something like this around others XP
Basically, Nightmare in the image flipped and twisted his head so that the top is now the bottom and his bottom jaw is hanging by his neck with his lovely smoke bellowing out. Don't worry, he can twist himself back together but he'd likely enjoy running around like this for awhile just to scare the tar outta fawns XP
Maybe then they won't be glued to him >>
.The river flows in you, you know that song? it is verrry nice 
