Yak Folk (CR 4)

Large Monstrous Humanoid
Alignment: Usually neutral evil
Initiative: +4 (Improved Initiative); Senses: darkvision 60 ft.

AC: 16 (-1 size, +7 natural), touch 9, flat-footed 16
Hit Dice: 5d8+10 (32 hp)
Fort +3, Ref +4, Will +6
Speed: 30 ft.
Space: 10 ft./10 ft.
Base Attack +5; Grapple +4
Attack: Quarterstaff +9 melee, or falchion +8 melee
Full Attack: Quarterstaff +9 melee, or falchion +8 melee
Damage: Quarterstaff 1d6+6, falchion 2d4+6/18-20
Special Attacks/Actions: Body meld
Abilities: Str 18, Dex 11, Con 15, Int 14, Wis 15, Cha 14
Special Qualities: Command genie, use staff
Feats: Improved Initiative; Power Attack; Weapon Focus (quarterstaff)
Skills: Disguise +10, Heal +6, Knowledge (arcana) +10, and Use Magic Device +10
Advancement: By character class
Climate/Terrain: Any mountains
Organization: Solitary, pair, or herd (3-12 Plus 1 leader of 1st-5th level and 1-2 humanoid or giant slaves per yak folk), or tribe (11-20 plus 1 leader of 5th-12th level, lieutenants of 1st-5th level, 1-2 humanoid or giant slaves per yak folk, and a force of 10-40 fighting slaves)
Treasure/Possessions: Standard plus staff (see below)

Source: Monster Manual II

Body Meld (Sp): Once per day a yak folk can merge its body with that of a humanoid or giant of Small to Large size. Except where noted here, this power functions like a magic jar spell heightened to 9th level (caster level 20th; Will save DC 21 negates). to use this ability, the yak folk must touch the intended target 20 minutes without interruption (yak folk usually restrain their victims while using this ability). At the end of this period, the target must make a Will save to remain conscious. On a failed save, the hosts mind becomes unconscious, and the yak folk assumes control. There is no magic jar per se - the yak folk physically merges its body with the victim's body without the use of an intermediate vessel. Body meld lasts until dispelled or dismissed. The process works only on humans, elves, dwarves, half orcs, and any creature of the giant type. The yak folk shares all the victim's knowledge, memories, skills, feats, and extraordinary abilities, but none of its spell-like or super-natural abilities. The yak folk retains all of its own super-natural and spell-like abilities. The merging does not shed a magical aura (so a detect magic spell does not reveal it), but a true seeing spell reveals the victim's dual nature. If the host body is slain, the yak folk dies with it. Separating the two bodies takes half as long as melding (10 minutes). The host regains consciousness 1d6 minutes after the separation is complete.

Only characters who know the victim personally have any chance to realize that something is wrong, by making a Spot check opposed by the yak folk's Disguise check (which in this case represents how well the yak folk impersonates its host).

Command Genie (Su): Once per day, a yak folk can summon and command a janni of evil alignment, but it can never have more than one janni under its control at one time. The janni is a slave bound to serve until the second sunrise after the summoning. The yak folk are greatly disliked by all genies, but for reasons lost in antiquity, no genie can attack a yak folk. Genies sometimes work to thwart the yak folk's plans or disrupt their lives in other, indirect ways, but even this is done cautiously because the genies know that if they antagonize the yak folk too much, the lives of enslaved jann will only become worse and their tasks more onerous.

Use Staff (Ex): A yak folk can use any magic staff This ability is similar to the Use Magic Device skill except that it applies only to magic staffs and the yak folk does not require a skill check. The chance that any particular yak folk possesses a magic staff is equal to its Hit Dice x 5%.

Most yak folk use their falchions or staffs, magical or otherwise, in combat.