There is an unused enemy in the game called Giant. This mob is a huge Zombie with considerable health and deals heavy damage. They can only be hurt by being hit in the legs. Giants were added in the last version of Survival Test, but haven't been used since. They can still be spawned through hacks, however. Additionally, they can be spawned using the command "/summon Giant" in version 1.7 of the game.
Contributed by Carlos