Take, for example, the zombie. I figured it wasn't available because zombies only have one skin variation, but skeletons also have one variation and they can be downloaded as a mini pack. I would just make my own resource pack out of the png file, but I haven't been able to figure out how to get the skin to show up in-game. Somehow I have the resource pack show up in the list but it doesn't affect the texture of the zombie :/