I imagine a zombified dragon to have torn skin with thick, dark red blood oozing out at each step. It's eyes would appear like pearls except without their luster. All flesh would have a greenish tint but also be pale. Perhaps in some places all flesh would be stripped away leaving only bone that was faded, yellow and worn. If this dragon had wings the skin covering the wing bones would be torn or, depending on its age, completely rotted away. If the skin still remained it would be substantially thinner than it had been during the dragon's lifetime.