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)
Linked issues
Comments 2
Possible duplicate of MC-91388.
Doesn't work for artificially spawned version(summon command/spawn egg). Must have to be spawned naturally.