I have noticed in the latest 1.12.1 Update, that regular Zombies occasionally spawn in The Nether instead of Zombie Pigmen.
To Reproduce The Error:
1: Enter The Nether
2: Have the Zombie Pigmen generate, and then kill them (for new Pigmen to spawn and possibly an error of a regular Zombie as well)
Edit: Sorry For The Duplicate Report!. I Searched 'zombies spawn in the nether and found no match for version 1.12.1. I didn't realize the Zombies were being summoned by the Pigmen, not naturally spawning