Minecraft wiki entry for Chicken_Jockey, associated text: Each baby zombie, baby husk, baby zombie villager, baby zombified piglin, or baby drowned that spawns has a 5% chance to check for an existing chicken within a 10×6×10 box centered on the baby's spawn location and spawn riding one of those chickens if there are any. If it fails that 5% chance to check for existing chickens, there is an additional 5% chance of the baby zombie spawning mounted on a new chicken. In a chicken-free environment, each spawned zombie has a 0.25% chance of becoming a chicken jockey; if chickens are present, the chance increases to 0.4875%.