Each baby zombie, baby husk, baby zombie villager, baby zombie pigman, 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 it will 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, this gives each spawned zombie a 0.25% chance of becoming a chicken jockey; if chickens are present, the chance increases to 0.4875%.
Chicken jockeys may spawn with items equipped. Baby zombie pigmen versions of the chicken jockey will always have their goldensword equipped while baby drowned versions may have a trident. Provoking the zombie pigman chicken jockey will cause it to attack, like a normal zombie pigman.