Oof yeah, that sucks... Chicken Jockeys are INSANELY rare... Also the 360 ver of minecraft doesn't have a disc for killing it so it was kinda disappointing that I wasn't playing a newer version of minecraft...
Chicken Jockey Spawn Chance:
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, 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%.