Burning Skeleton (CR 3)

Medium Undead
Alignment: Always chaotic evil
Initiative: +2 (Dex); Senses: Listen +9 and Spot +9

AC: 16 (+2 Dex, +4 natural), touch 12, flat-footed 14
Hit Dice: 5d12 (32 hp)
Fort +1, Ref +3, Will +5
Speed: 30 ft.
Space: 5 ft./5 ft.
Base Attack +2; Grapple -3
Attack: Slam +4 melee
Full Attack: Slam +4 melee
Damage: Slam 1d6+1 and 1d6 fire
Special Attacks/Actions: Fire
Abilities: Str 12, Dex 14, Con -, Int 10, Wis 13, Cha 10
Special Qualities: Undead, immune to fire
Feats: Alertness; Weapon Finesse (slam attack)
Skills: Climb +5, Hide +9, Listen +9, Move Silently +10, Search +7, and Spot +9
Advancement: 6-10 HD (Medium-size)
Climate/Terrain: Any land or underground
Organization: Solitary, gang (2-5), or pack (6-11)
Treasure/Possessions: None

Source: Return to the Temple of Elemental Evil

These undead approach combat in a straightforward way.

Burning skeletons are undead that lurk around areas of fire -- volcanoes, forges, fire pits, and so on. They are usually animated spirits of those who died in flames who have returned to wreak havoc. Left to their own devices, they seek to destroy all life and burn anything they come across.

These undead resemble standard skeletons, although the bones are usually charred. An aura of flickering flames, particularly about their hands and skulls, is always evident, and their eyes glow like the ends of hot pokers.