Spider Jockey

A Spider Jockey is the rarest known hostile mob in Minecraft. When this mob spawns, there is a very rare chance (1%) that a skeleton will spawn with it on a spider's back. This can be a very deadly combination, as spiders move very fast and can climb walls, and skeletons can shoot from far range. This mob is treated as two mobs, so when one dies the other survives. If you are beneath the spider, the skeleton will shoot the spider instead of the player. In creative mode a spider spawn egg has 1% chance of spawning a spider with a Wither Skeleton riding it instead of normal skeleton, but only in the Nether. Skeletons will spawn if a Spider spawn egg is used in the Overworld.