Well Milkyjon really broke down all the possible tips for improvement, I just have to note it's a really amusing character :)

And I really like it despite all of it's technical imperfections (those will fade away with some practice real quick)

I like the design, but as shadow wolf said it has the appearancec of 3 layers tweened ontop of each other. It would have been nicer if you would have had the jaw fixed in one joint and drew it a couple of frames opening and closeing. Some self shadowing behind the skull aswell to make itb sit nicer. I also noticed a couple of pixels jumping about on the right of the jaw. I would try to keep your outline shades consistent in dark/lightness and choose a fixed lightsource and stick to it. @chafan-I think the thing in the middle of his legs is infact his right front foot which is scraping the floor, dont think its directly side on som you cant see his other back foot.

Sorry for the rant. Nice design, keep it up.