That Zombie Pigman looks a little strange. Oh, wait... That's no Zombie Pigman, that's a regular Zombie! But what's it doing in the Nether?

Concept: The Nether Prisoner would have less HP than Zombie and would also be a lot weaker. There would be a secret room in the Nether Fortress where these mobs are kept, usually in groups of 3 to 5 entities. Once it's free, the Nether Prisoner would start attacking every Nether creatures it sees (unless the player hits it themselves, which would make the whole squad mad.) When the Nether Prisoner dies, it would drop one of these items:
* 0 - 2 Rotten Flesh (obviously);
* 0 - 3 Nether Wart;
* 0 - 1 Amanita Muscaria;
* 0 - 1 Gold Ingot (rare);
* 0 - 1 Poisonous Potato (rare).
Background: At the certain point of the story's timeline, a group of Zombies got themselves into the Nether dimension. Unable to remember their way back to the portal due to their rotten brains dehydrating as they step further, they eventually got completely lost. Though later on they were found by an unknown Nether civilization and banished into Fortress' cells. They had no idea what kind of creatures they're dealing with, so they fed the with literally anything they had (including gold.) During the imprisonment, the Zombies had slowly degraded into Nether Prisoners, empty vessels with no souls and no memories who don't even know who to attack anymore.

Make sure you check your Nether Fortress for secret rooms next time!