A Chicken Jockey is a very rare hostile mob, which consists of a Baby Zombie or Husk riding a Chicken. The chicken uses the rider's A.I. Instead of being passive, the mob is hostile, and it moves just as fast as the Baby Zombie or Husk would. However, if you kill its hostile rider, the chicken will become passive. These mobs are very similar to the Spider Jockey. The chances of finding one are around 1%. There is also a Pigman Jockey (Baby-Pigman riding a Chicken) and a Villager Jockey (Baby-Villager zombie riding a Chicken).
Chicken Jockeys are very rare Mobs that appear as a Baby Zombie or a Baby Zombie Pigman riding a Chicken. They were added in Update 0.11.0.