The first thing you might want to do here is draw out the eye starting with the brow. When that is done draw the half circle shaped eyeball, and then tackle the task of getting the mouth drawn out. As you can see I exaggerated a bit by making the teeth sharp instead of squared. This is because the zombie pony is a mutated version of the real thing. Add the gum-line as well as torn flesh lining around the mouth. Lastly for this step, make the nose and some detailing marks on the face along the forehead, snout, and inside of the ear.