Undead one

The Undead ones play a critical part in the Shilo Village quest. They are found in various dungeons visited during the quest and in a fenced off area just outside the village. This makes them decent training for people training Range, Magic, and meleers wielding halberds. When killed, some release a gas that can damage 20-30 life points. However it is not poisonous, so antipoison is not required. The gas only affects characters in a 1-square radius around where the undead one was when it was killed, so if you kill one you have a few seconds to run to avoid taking damage.

They can be used as a substitution for Zombies as a Slayer task for higher leveled players who wish to get more experience from their task, or those who use Duradel/Lapalok. They do not drop anything, though, and so are rarely killed.

Trivia
Some of the (level 73) undead ones carry a macuahuitl, a type of weapon used by the mesoamerican Aztecs.