Minecraft wiki entry for Skeleton_Horse, associated text: A skeleton horse's health is always 15. Displayed hearts are health, divided by two, rounded down. A horse with an odd number of health points (15, 17, 19, etc.) does not show the last half-heart. If the horse has lost one health point lower than the inflicted damage and did not regenerate, it has an odd number of health points, otherwise, it has an even number of health points.