Bone Dragon is an enemy fought at Lair of the Father and Lunar Subterrane. Despite being tagged as Undead, it is resistant to Holy-elemental and Cure attacks until the 3D remakes. It is not a difficult foe to fight, and can be easily defeated with the Excalibur, Ragnarok, Leviathan, or Bahamut. It attacks using the Blaze skill.
Bone Dragon refers to the purple version of the enemy, fought during the Crystals Tale at the Subterrane and the C-Peninsula. It is otherwise identical to its previous counterpart.
Skull Dragon is a boss first fought at Kefka's Tower, then later in the Dragons' Den in the Game Boy Advance and later remakes. In the first battle, it attacks primarily using status attacks such as Doom, Will-o'-the-Wisp, and Disaster. In the second battle, it can only be defeated by depleting its MP pool completely, and will attack using more status effects such as Fear, Level 5 Death, and Disaster.
Skulwyrm is an enemy fought first at the Zertinan Caverns, and later at the Great Crystal. They attack using Sonic Fangs and Crushing Fangs, and when fought in the Great Crystal, also uses Curse and Disablega. They are the only source of Wyrm Bone in the game, making them highly valuable for obtaining the Scorpion Tail.