Minecraft wiki entry for Horse, associated text: Horse's hearts ranges from 15–30, with an average of 22.5 × 11.25. 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. One may test this by hitting a horse: if the horse has lost one-half heart from one punch, it has an even number of health points. Otherwise, odd.