Abyssal bats, now in six delicious flavors!
Type I (Rykso): Cold!
Type II (Riptyce): Fiery!
Type III (Corteel): Electrified!
Type IV (Caid): Acidic!
Type V (beserking!) and Type VI (spellcasting!) are "greater varrangoin", with significantly greater power and intelligence. Their true names are kept secret lest they be used against them.
* A wizard attempting to summon a "Type III demon" accidentally gets several Corteel Varrangoin instead. The perils of using these poorly translated ancient grimories from the Prime Material Plane. While his wards and bindings would have been fine against a Glabrezu, they were ironically useless against a flock of lightning-bolt-breathing hell-bats, and now he needs some PCs to help clear out his tower.
* The ritual scarification of dots, curves and lines all varrangoin have are believed to form the spellbooks in the case of the greater varrangoin spellcasters. An eclectic geometer-wizard wants to expand his understanding of spell-encoding, and thus hires the PCs to capture several of these spellcasting monsters (with a significant bonus for any captured alive)... but in addition to the abilities of a 9th level wizard, they've got significant spell-like abilities including but not limited to dispel magic, polymorph self, and polymorph other. And there's the annoying fact that they're fairly hard to tell from every other kind of Abyssal bat.
* Lesser varrangoin collect treasure they find pretty: gems, jewelry. Unfortunately, the woman complaining about her stolen brooch is actually a tanar'ri in disguise, hoping the PCs will find a few decent nests in their quest for it that it can force into magical servitude.