Jockey

A Jockey is a rare mob in which a Hostile Mobs spawns riding another Mob. They were first added in Update 0.11.0.

Spider Jockey
A Spider Jockey spawns as a Skeleton riding a Spider or a Cave Spider. It is a very dangerous mob as the Skeleton uses ranged and the Spider uses melee, making it a deadly combination.

Chicken Jockey
A Chicken Jockey spawns as a Baby Zombie or Baby Zombie Pigman  riding a Chicken. It moves very fast, making it very hard to outrun. Only the Zombie is hostile, not the Chicken.

Wolf Jockey
A Wolf Jockey spawns as a Baby Zombie riding a Hostile Wolf. It will be added in Update 0.14.0.

Cow Jockey
A Cow Jockey spawns as a  Baby Zombie  riding a Cow. It will be added in  Update 0.14.0.

Trivia

 * Cow Jockeys and Wolf Jockeys are exclusive to Minecraft Pocket Edition.