Minecraft wiki entry for Spider_Jockey, associated text: A cave spider can spawn as a jockey, just as a spider can. If either type of spider is spawned in the Nether and occurs as a jockey, it has an 80% chance for its skeleton rider to be a wither skeleton. If cave spider is spawned in a snowy biome with a view of the sky, and occurs as a jockey, it has an 80% chance for its skeleton rider to be a stray.