Belker is an elemental smoke monster from Monster Manual for D&D 3.5. I've never used this monster, and I think I'm not the only one to overlook this creature. Its signature ability is to transform into a smoke cloud and solidify the smoke inside creatures' lungs. While Respiratory Agents usually cannot be applied to cutting attacks, I thought that I can make an exception for this ability.
In my TL 4 fantasy setting, a heavily forested region is ravaged by plagues and monsters left after a magical equivalent of a biological war between the wood elves and the neogi - alien spiderlike slavemasters. While the open warfare is over, there are still covert operations to spread new diseases, release monsters, and free slaves. Some of the wood elven slaves did not need any help setting themselves free - they threw off physical and mental shackles of their neogi slavemasters through sheer willpower and the power of their unique martial art they have developed in the slave pits - Guali Pianil, or "chitin breaker" in Wood Elven. While the style is useful against the neogi, it is useless against the umber hulks - you can only run from them.
The beholder is an iconic D&D monster with powerful magical abilities. Gauth, also known as the lesser beholder, is much weaker, but still formiddable. In GURPS, it seems, he's fall quickly to ranged combatants.
Behir is a serpentine draconic monster that is very intolerance of actual dragons. Dwelling in warm hills, this beast is intelligent, but not very bright. It is tough, strong, and possesses an electric breath attack that takes recharge.
A barghest is a lupine fiend that can take the shape of a wolf or a goblin. In its natural form, it resembles a goblin–wolf hybrid with terrible jaws and sharp claws. Barghests come into the world to feed on blood and souls and thus grow stronger.
Soulspark is a mote of independent incarnum energy. A weak small monster that nonetheless may pose threat to some adventurers.
The chain devil is one of the most interesting devil types in D&D, in my opinion. They fall out of the traditional tanar'ri hierarchy, and in Pathfinder they are a whole separate outsider type altogether. A kyton is physically strong, but not particulary bright. He's always covered in a layer of chains, which provide him unreliable DR in addition to his naturally resistant skin. Use of holy and/or silver weapons is advised. The chains effectively grant the chain devil a pair of kusaris - Reach 1-4 flail-like swinging attacks. They hit hard and are hard to parry. What makes things worse is the kyton's ability to animate other chains, allowing him to strike distant enemies. If the enemies wield chains, then it makes things even worse. In addition, making his face resemble the face of a loved one or the worst enemy can be very unsettling.
The devourer is a powerful undead monster spawned by the extraplanar githyanki (see Dragon Magazine #355). Placed in the middle of the CR spectrum, it should prove to be a challenge for a middle-level party. While converting it to GURPS I merged energy drain and essence steal into a single draining ability that powers the spellcasting, evading including an instant death attack. Giving the monster some Wrestling with high ST should make it easier for the devourer to grapple the victim and drain his life.
