The horses' jaws are 2D, and 2D elements usually duplicated by inverting the duplicated side... but in your case, the jaw is completely broken: some polygons, so to speak, face one way, and others face the opposite way.
Added 3 hours 27 minutes later
In general, I don't know how to do this correctly yet. If I copy and flip the polygons, I can't seem to do the skinning for them afterward. And I don't know how else to do it.
