A pigmanjockey is not visible on a chicken.
This video will show you what I mean:
https://www.youtube.com/watch?v=HjUyy-z5jEc
UPDATE
when you log out, or unload the chunks and then load them back in, the baby pigman will be there! (After you kill the chickens)
Related issues
Comments
migrated
migrated
Possible duplicate of MC-91388.
Doesn't work for artificially spawned version(summon command/spawn egg). Must have to be spawned naturally.