Java Edition:Adult Zombie Villager Chicken Jockey
Normally, only baby zombie villagers can ride chickens. However, there have been methods of getting an adult zombie villager to ride a chicken instead.
Obtaining
Curing and Infecting
Due to a bug[1] from 1.14 to 1.16.2 Pre-release 1, zombie villagers would dismount minecarts and chickens (baby zombie villager exclusive) when cured. However, since the bug was fixed, curing a zombie villager no-longer dismounts it from its ridden object/entity. As a result, from 1.16.2 Pre-release 1 to 24w38a (present), if a baby zombie villager chicken jockey is cured, then the baby zombie villager will turn into a baby villager chicken jockey.
This discontinued feature is derived from the adult villager chicken jockey which is then zombified. Adult zombie villager chicken jockeys can be normally only be obtained in the Overworld as zombies naturally spawn in the Overworld. However, by placing a spawner, and converting the spawner into a zombie spawner with a zombie spawn egg this can be obtained in the Nether and the End.
22w13oneBlockAtATime Mob Stacking
In the April Fools' snapshot 22w13oneBlockAtATime, the player is able to pick up nearly any entity in the game. Picking up these mobs will stack them on top of the players head, each added mob will be riding the mob before it, the player is then able to throw the stacked mobs to let go of them. This can be used to obtain an adult zombie villager chicken jockey, however chickens are easily killed doing this so its advised to create a stack of 3 chickens.