Regular Zombie found in the nether

What I expected to happen was...:
I was not excepting a zombie to appear in the nether

What actually happened was...:
One zombie to appear in the nether with chain armor.

Notes:
Hitting regular zombie will result zombie pigman turning hostile to the player.
It was tested on the nether in survival mode.

Summary:
When I went to the nether fortress. I found a zombie far away from the nether portal. I was surprise because zombies are not found in the nether except chicken jockey, and zombie pigmans.

Steps to reproduce
1.Not exactly how the zombie spawn in the nether. It was either a spawn error, or the chicken jockey spawn have gone wrong
2.You have to wait for zombie to spawn probably like 1 to 5 hours
3.It spawn is probably the same as chicken jockey spawn rate 5%