The Bone Dragon, also known as D. Bone and SkullDragn, is an enemy from Final Fantasy IV. For a currently unexplained reason, this monster has had the undead tag since the game came out, but seems to have been resistant to Holy and Cure until the DS re-release.
It is not a tough enemy at the stage of the game that it appears in, and using Excalibur and Ragnarok will completely destroy it. Leviathan and Bahamut also completely destroy it with no problem. Its attacks are of no threat.