In one thunderstorm I was lucky enough to find two skeleton trap horses. I killed the skeleton riders spawned from them and proceeded to take the horses back to my settlement. The first set died off before I could get back. As for the second set, one died off but we managed to get the other three back and heal them, BUT they are still dying. Now we are stuck trying to keep them alive as opposed to doing other things because of their rarity and the odds of finding more. Any kind of clarification as to why they slowly die would be greatly appreciated. Also, I don't know if it helps, but it is a strictly survival only world.