The following is a list of undead monster types from Diablo III, though it's important to keep in mind that some may have changed radically or been removed altogether during development:
Ghoul - Ghouls are typically fast-moving, cannon fodder-type monsters that swarm players in large numbers. Unlike the Risen Dead (see below) they're fast and dexterous, able to climb up walls and leap out of the shadows. Their strength is fuelled by feasting upon human corpses.
Grotesque - Grotesques are fleshy undead creatures that can absorb a lot of damage. Once defeated, they explode like a pinata, and worm-like creatures called Lamprey burst out of them.
Lamprey bursting out of a Grotesque.
Morlu - the Morlu are undead warriors that serve the Lucion and the Triune. They're tall, heavily armored elite troops with a strong lust for battle that can even turn upon themselves if there are no better targets in sight.
Risen Dead- better known as zombies, these are your traditional slow, lumbering undead monsters, though they do come in a number of different varieties, including crawling torsos and wretched undead, which have the ability to summon other undead creatures by feasting upon the remains of bodies and regurgitating them.
Skeletons - Skeletons are the re-animated bones of dead humans that have been filled with dark energy. Traditional skeleton warriors will be joined by Skeletal Archers, Summoners, Shieldmen and Executioners.
Unburied - Unburied are large, spike-covered undead monsters that are created wherever bodies are dumped together in mass graves. Much like the Thorned Hulks from Diablo II, they are huge, lumbering creatures, though their melee attack is both fast and dangerous.
Unburied concept art.
Wraith - Wraiths are incorporeal beings that come in many different forms and which possess the ability to drain character attributes, such as life and mana.