Use the summon command for a baby zombie with a chicken as its mount
Java Edition: `/summon zombie ~ ~ ~ {IsBaby:1b,Passengers:[{id:”chicken”}]}`
Bedrock Edition: use a spawn egg or command block setup, since direct passenger NBT commands are not supported the same way
If needed, spawn a baby zombie first, then place a chicken under it so it mounts automatically
In creative mode, use commands or a custom mob spawner to create the setup
In survival, chicken jockeys can spawn naturally very rarely with a baby zombie riding a chicken