Skeletons are Undead creatures that may appear as a skeleton of a Human or some other creature. A Viviblix could also animate the skeletons of the dead. Mr. Lich was shown to be able to animate multiple Human, and non Human skeletons, including the skeleton of a Triclops and the Dragon Ranticus.
In Wrath of the Dragon King, at least one was involved in the contest at Stormguard Castle. The skeleton controlled the boat that led to the Banishment Rod's room and would take contestants at the cost of one of the magical coin (a coin that was previously aquired during the contest).