Minecraft wiki entry for Summoning_jockeys, associated text: It is always easier to defeat a jockey if you attack the mount before the passenger. Knockback will have no effect whatsoever on the passenger. However, setting the mount on fire will also set the passenger on fire. If a mount is a hostile mob (such as an skeleton), it can still shoot arrows at the player if the passenger is facing the player, as mount can turn on its own, but it takes a while due to its very small "resistance".