Minecraft wiki entry for Summoning_jockeys, associated text: However, if there are multiple mobs of the same genre this rule can cancel out. Let say you have 3 zombies stacked on top of each other and a cow on the very top. Well, if there are 5 zombies that's 5 x 0.25% of the speed, which means the zombies can go as fast as they want, but the cow still has dominance over movement and can easily move a different direction, but the second the cow stops moving the zombies will resume their intentions.