Bones are found in groups together or with Crawls and/or Red Bones. They can be found in and near the Temple of Fiends, and in the Marsh Cave.
Belonging to the Undead category, they are weak to the Sun Sword, Light Axe, and Xcalber, and subject to damage by Harm spells. Fir2 or Harm work wonders on them.
In the original Japanese version (and FF Origins) Bones are known as Skeletons. Of course, a skeleton is the system of bones found in a body (hence the English name). The idea of reanimating them into a soldier most likely comes from many cultures. Skeletons return in FF3, 4, 5, 9, Mystic Quest, and Tactics.