Bone wraith

A bone wraith is a skeletal monstrosity fused together from the bones of the undead.

Similar creatures known as boneguards are found in the Shadowlands.

As a companion pet
Fragment of Frozen Bone has a chance to drop from Lord Marrowgar in Icecrown Citadel.

Speculation
At first, it was believed that the bone wraiths were creations of the Lich King, since the only one known was the fearsome Lord Marrowgar. However, with the recent resurfacing of Uldum and Halls of Origination it seems to be more of a "natural" occurrence, like many other undead, as Earthrager Ptah cannot possibly be affiliated with Scourge in any way. It is also possible that it's just a coincidence, and more a technical than a lore-related matter (read - model reuse) on Blizzard's part. It could also be speculated that the Lich King (or Arthas Menethil) could have been inspired by the tales of bards or folklore in Lordaeron and/or Azeroth about these constructs, and chose to create or perfect it himself.