The following is a list of demon types from Diablo III, though it's important to keep in mind that some may have changed radically or been removed altogether during development:
Activated Vessel - these are human cultists who have allowed themselves to be taken over by a demonic presence, distorting them into mutated monsters.
Arreat Crater Flyer (working name) - these are the large-winged demons that are seen taking part in the demonic invasion of the northern Barbarian lands near the Arreat Crater.
Berserker - berserkers are melee demonic beasts, with huge fangs and monstrous faces, that serve the Cultists. They are effectively the Cultists' army.
Catapult - catapults will make a return from Diablo II. They're the enemy siege engine and will likely be seen during the assault on the Barbarian lands by Azmodan in Act III.
Crater Assault Troop (working name) - these have been seen in various cinematic trailers - where they're fighting Barbarians - and pieces of concept art, and will probably make up a large part of Azmodan's ground forces.
Crater Assault Troops.
Dark Demon - dark Demons are dog-like animal demons summoned by Cultists from red manhole-like openings in the ground.
Dark Vessel - Dark Vessels are human cultists that are in the process of being taken by a demonic presence, and are commonly seen in circles of four to eight cultists performing the ritual. The Vessel itself is harmless until it turns into an Activated Vessel.
Dune Dervish - the Dune Dervish are creatures that resides in the deserts of Act II. Their trademark move is a whirling attack that does damage, knocks the player back, and reflects most types of projectiles.
Fallen - the Fallen Ones of Diablo II return. Though they remain small and generally cowardly, they have been upgraded in appearance and tactics. Their strength is also bolstered by more powerful chieftan units, and the Shaman and Overseer types that can command the common Fallen into a powerful group.
A Fallen Imp.
Flyer - flyers are large, bat-like creatures that infest many of the dungeons in the game.
Frozen Mage (working name) - Frozen Mages are reminiscent of the Mage enemies in Diablo II, though they aren't strictly related. They're known to inhabit tombs underneath Act II.
Imp - Imps are small, humanoid demons found in the early dungeons of Act I. Weak alone, they're fast moving, small, annoying ankle-biting enemies that tend to attack in large packs.
Malformed - these are mutant, deformed Goatmen of various types (at least two different types are known about - spear throwers and heavy brawlers).
Malformed concept art.
Poison Mage (working name) - they're similar to the Frozen Mages, though they have green blood and poison projectiles.
Sand Golem - Sand Golems are new to the series and presumably will show up in Act II of the game.
Tomb Viper - Tomb Vipers are serpent men. The design is clearly inspired by the Claw Vipers of Diablo II. They're primarily melee attackers that wield tridents.
Treasure Goblin - Treasure Goblins are a wholly new concept for the Diablo series, since they seem to pose no danger to players. They run away from the hero, dropping gold as they move and as the hero attacks.
Triunes - Triunes belong to a cult which worships the three Prime Evils and seeks to recruit men to pledge allegiance to Hell.
Triune Leader Blood Guard concept art.
