At first glance, it's just a zombie. So what makes it a Zombie Giant? Well, take a closer look.
See its head? See how there are a couple of things in there? Those are regular-size zombies. They're riding in the Zombie Goliath's cranium! Now the flavor text makes more sense:
"Removing the encumbrance of useless brain tissue grants several advantages: a slight increase in mobility, a response of revulsion and horror from the enemy, and, in the case of large specimens, room for passengers."
—Zul Ashur, lich lord