By the way for those who are wondering, the chance for a normal Zombie Chicken Jockey to spawn is a 1 in 2000 chance. Considering that this is a Zombie Pigman Chicken Jockey, the chances cound be even lower.
I;ve had this a few times to be honest n_n Mind you... my minecraft statistics say I have killed 636,307 zombie pigmen and I've died to them 3 times... GGWP
I saw one run out of my grinder the other day and slowly flutter then 150 blocks to the floor. It was so awesome
These are actually quite common. I leave my grinder going for an hour and i get about 20-30 of those.