Minecraft wiki entry for Spider_Jockey, associated text: There is a 1% chance for a spider to spawn with a skeleton riding it, forming a spider jockey. In the Nether, a spider (spawned with a spawn egg, commands or spawners, since these do not naturally spawn there) has a 0.8% chance to spawn a wither skeleton on its back and form a spider jockey. In snowy biomes, a spider has a 0.8% to spawn a stray on its back and form a stray spider jockey instead.