Minecraft wiki entry for Skeleton_Horse, associated text: A skeleton trap horse is a hostile skeleton horse spawned from a fraction of lightning generated naturally by a thunderstorm (0.75–1.5% chance on Easy, 1.5–4% on Normal, and 2.25–6.75% on Hard, depending on regional difficulty). Lightning strikes caused by a Channeling-enchanted trident and lightning strikes on a lightning rod never spawn a skeleton trap horse. The trap horse despawns after 15 minutes if not triggered.