Question: where do skeleton horses spawn?
Answer: Skeleton horses spawn only from "skeleton traps". A "skeleton trap" horse is a skeleton horse spawned from a fraction of lightning strikes during 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). It despawns after 15 minutes if not triggered.