Can zombies spawn in the nether?

My nether portal in the overworld is light up and gated to prevent mobs entering/leaving, but I recently was attacked by 10 or more zombies when I entered the nether. No skeletons, creepers or spiders.

Could it be a glitch where the pigmen are spawning as zombies? I killed 4 pigmen before the zombies appeared, and 2 were wondering the area after I slaughtered the hoard.

A big wave of pigmen with a single zombie attacked. Then a zombie villager appeared behind me in my small pigman hunting lodge which contains the portal. A couple more zombies appeared outside. This happened twice but the second time there were more zombies and less pigmen in the initial wave. I also had another wave without a zombie in my bunker.

I think it might be an event like the zombie hoards that attack villages.

Zombies have been dropping from higher platforms that can't be accessed from the portal platform and I have stayed close to the bunker so I doubt they have been coming through it. Especially because there has not been a single skeleton, creeper or spider.

Update. I just saw a zombie spawn beside a pigman in a lit area. I had not hit any pigmen yet.

Snocrach also said:

Zombie pigmen can summon their zombie friends from the overworld to help them fight the forces of evil.

this is a normal minecraft feature - nothing to do with the world generation.

I am playing the Snocrash's Hermit Chunk Challenge in 1.9

Solution 1:

I am not sure if this issue is fixed, but Zombie Pigmen can or could spawn Zombie reinforcements when attacked. It was intended that they would spawn Zombie Pigmen as reinforcements, but since the code is kind of related, I believe that could happen. It's possible that's still a thing in 1.9.

Solution 2:

It is strange, but Zombies can actually spawn in the Nether, possibly due to a bug or intended coding.

But it is very odd that there were 10 Zombies. Zombies rarely spawn in the Nether, and only if you get "lucky" there will spawn 4 at once.

Probably that multiple groups spawned (this happens very rarely), and the Zombie Villager spawned because of the 5% chance for a Zombie to become its Zombie Villager counterpart.

Also, additional Zombies could have spawned due to the Zombie Reinforcement feature, which means that if you hit a Zombie, others may spawn and head to attack you.