A spider has a 1% chance to spawn with a skeleton rider. This means that a spider spawner, spawn egg, or the /summon command can very rarely spawn a spider jockey. Spiders spawned using spawn eggs in the Nether have a small chance (0.8%) of a wither skeleton spawning on them, creating a wither jockey. Likewise, due to the fact that 20% of all skeletons spawned in the Nether are ordinary skeletons, a skeleton may be on the spider the other 0.2% of the time.