Fiendish creature

A fiendish creature was a creature living on one of the Lower Planes that bore a resemblance to a similar creature on the Material Plane. Often mistaken for half-fiends, these creatures were not nearly so powerful and did not share a bloodline with any of the true fiends. Many fiendish creatures were to fiends as animals were to humans of the Material Plane.

Description
Fiendish creatures looked much like their counterparts on the Prime, except that they often had visible signs of the unholy realms where they dwelt, with a somewhat terrifying appearance.

Abilities
Outside their fearsome appearances, fiendish creatures were in most ways similar to the prime creatures that they resembled, but there were a few significant differences. All fiendish creatures were sentient with at least minimal intelligence. A fiendish snake would be more intelligent than any snake native to Toril.

All fiendish creatures had the ability to see in the dark and were resistant to harm from cold, fire, spells, and often to attacks from mundane weapons.

An attack from a fiendish creature had the supernatural power to cause more harm to creatures that were good.

Personality
All celestial creatures were evil like the planes on which they dwelt.

Ecology
There were a plethora of creatures living on all of the Lower Planes that were similar to ones from Toril yet had adapted to those evil worlds. For example, fiendish crustaceans inhabited Blibdoolpoolp's realm in the Fated Depths. Fiendish dire wolves and trolls hunted the layer of Niflheim in Hades. Fiendish sharks swam the waters of Stygia.

Notable Celestial Creatures
Over the centuries, both divine and arcane spellcasters had learned the power to summon many varieties of fiendish creatures from their home planes for temporary aid. Common among these were fiendish apes, boars, crocodiles, deinonychuses, dire rats, dire weasels, elasmosauruses, giant praying mantises, giant spiders, giant wasps, girallons, hawks, megaraptors, monstrous centipedes and scorpions, octopuses, ravens, rhinoceroses, sharks, snakes, squid, tigers, tyrannosauruses, wolves, and wolverines. In the case of divine magic, these creatures were most often servants of the deific power worshiped by the summoning cleric.

The spell vipergout also summoned fiendish vipers.

Narzugons were known to ride fiendish dire boars into combat.