When punching a horse that hasn't taken any damage yet and was spawned by commands or spawneggs the player deals somewhere between 20 and 30 hearts of damage to the horse. This also affects other similar entities like llamas, donkeys and mules. The problem seems to be that the health and the max_health attribute in the nbt data don't match because the health is always 53 while the attribute shows the amount of health they should have if they would have spawned naturaly.