Minecraft wiki entry for Spider, associated text: There is a 1% chance for a spider to spawn with a skeleton riding it, forming a spider jockey. In the Nether, if a spider is spawned there with a spawn egg, commands or spawners, it has a 0.8% chance to spawn a wither skeleton on its back and form a wither jockey instead. In some of snowy biomes, a spider has a 0.8% chance to spawn a stray on its back and form a stray jockey instead. A spider jockey can also be spawned by the command /summon spider ~ ~ ~ {Passengers: [{id:skeleton}]}.