We've had plenty of undead pets, and some of them have been humanoid in nature. However, there have been a couple models that would be great pets that haven't been implemented.
Zombies are lesser forms of ghouls, which are simply reanimated corpses. They have very little (if any) will of their own, and mostly shamble forward to their victims. They usually aren't very large, being about the size of a human, or smaller. If a zombie pet was implemented, I would hope it would be a little smaller than the Ethereal Soul-Trader, while still being a good size that is believable as an animated human body.
It could drop from Precious and Stinky who both summon tons of these sort of adds when you fight them. As a battle pet, it could have moves like Bone Bite, Rot, Consume, Absorb, Bone Barrage, and Stench. The pet could have an associated achievement for using it to defeat an enemy Singing Sunflower as well.
Goblin Zombies were really only seen on the Lost Isles, where pygmy witchdoctors were sacrificing the Bilgewater Goblins for Volcanoth to eat. Since there wasn't really a reason to use the model again, they never did. However, I'd like to see it again as a pet.
The pet could be obtained from a new quest near any neutral goblin zone - Gadgetzan, Fuselight, Mudsprocket, etc. The graveyard could have a new quest giver, who could be a ghostly goblin who lost his body and can't make his way back to life. You're sent to go out and find the pieces of his body, which were taken by the local mobs. Kill the mobs, get the pieces from their loot tables, and come back to the ghost. You put together the body, and it comes to life without the goblin's soul. He shrugs and say you can keep the body and use it for whatever you want. The quest reward is the [Zomblin].
As a zombie pet, it could have several of the same moves as the Harmless Zombie. Some of the moves could be replaced with mechanical moves, like Missile or Make it Rain - it would after all be a goblin!
What do you all think? Let me know on Twitter! Thanks to Wowhead for the pictures from their model viewer!