In 1.15.2, zombified variants of mobs have been something like Zombie Pigman or Zombie Villager.
In 1.16 and later, however, Zombie Pigman was renamed to Zombified Piglin. However, this does not affect other mob names, making them inconsistent.
Here is what it SHOULD be
Zombie Villager = Zombified Villager
Zombie Horse = Zombified Horse