Skeletons are the re-animated bones of slain humanoids. They can be summoned by mages (particularly necromancers) and clerics, though they may also be found in almost any area around the Sword Coast, Amn and Tethyr, especially cemeteries, tombs and High Hedge.
Be wary of their resistances. They are highly resistant to piercing and slashing damage, nigh indestructible by missiles, and completely immune to cold. Blunt weapons, such as maces or staves, and magic work best.