Minecraft wiki entry for Summoning_jockeys, associated text: Always remember that no matter what, the passenger has complete control over movement of the mount. If you summon a cow riding a zombie the zombie can try to slowly move towards the player and attack him but the second the cow moves the Jockey will move wherever the cow desires with no resistance. The only exception to this rule would be flying mobs. Bats, ghasts and blazes cannot fly if they are passengers, but they can move the mount at higher speeds. If you place a ghast onto a spider, the ghast will strafe or move forwards (as it does when it's attacking).