Minecraft wiki entry for Chicken_Jockey, associated text: In Bedrock Edition, if a baby zombie has spawned on another type of mob (such as an adult zombie), killing the baby zombie's current mount in an area populated with chickens may result in the baby zombie jumping onto a chicken rather than attacking the player. In addition, Baby Zombies riding on other zombie variants appear to have a chance to change into the baby variant of the mob it's riding.