Minecraft: Baby Zombie Pigman, without the zombie pig part of the head (just the pig skeleton under the hat part of the head thing) riding a chicken, naturally spawning...

Im not sure if this is a bug, or something now in minecraft... but I was messing around with spawning things with spawn eggs on 1 block (spawning a bunch on 1 block so that they spew everywhere when too many of them spawn) and...... I spawned a bunch of zombie pigmen, and... they spewed everywhere, and then I spawned another after that, but stopped, when I saw this... a baby zombie pigman, that didnt have the pig part of the head, it just had the skeleton part under that, just like snowmen have a face under the pumpkin, they have a skull under their zombie pig face.... but this one spawned with out it... and it spawned riding a chicken... I AM SO CONFUSED!!! WHAT IS GOING ON HERE?????? and no I didnt use the /summon command... I dont even know how to use the {insert some random things that do things here} thing... so, i dont know how to summon things riding other things, and I thought that when something was riding something else, it was kind of like, ontop of it? this one was... kind of INSIDE the chicken... I spawned another regular pig-man next to it to show the... um... facelessness...