The Chimera, as found in Insomniac's Resistance series, simply gross me out. They're the closest to "cool" on this list, but there's something just utterly icky about them, and that makes for effectively detestable villainy.
Their stretched mouths and multiple eyes give them an insectoid look, but they're built like humans to create an unquestionably unnerving effect. Like the Locust, it's the fact that they make an ugly appropriation of human features that effectively amplifies their hideousness. They look more like animals than sentient beings, yet they walk on two legs, operate machinery, and even wear pants. It's the pants-wearing that really does it.
Animals in pants are simply creepy.
The Elder Scrolls universe is home to many weird and wonderful races, but among them, the most terrible are surely the Daedra. Although a sketchy designation, since Daedra relates to any otherworldly or "demonic" creature that comes to the realm of Tamriel through whatever mysterious means, those creatures in thrall to the five Daedric Princes are all so awful looking that they deserve a mention.
From the humanoid Dremora to the bizarre crocodilian beast known as the Daedroth, there is not a single creature in Oblivion that could be described as even halfway pleasant to look upon. The closest to attractive that a Daedra gets is a Mazken, otherwise known as a Dark Seducer. Even then, they're dark purple and look more like murderous grapes than anything else.
Few enemy races typify "vile" more than the Combine from Half-Life 2. Visually and morally, these parasitic miscreations are awful in every sense of the world. The leaders, and arguably the true Combine, are the Advisors, who are best described as looking like an Alien Queen's egg sac with a worm for a tongue, having been dipped in mucus. In other words -- not something you could take home to show the parents.
Being little more than a quivering, bulbous mass of telekinetic flesh, they are so far removed from humanity as to be truly nightmarish. While other creatures such as the Locust and Chimera derive their horror from how humanoid they are, the Combine Advisors run as far as they can in the opposite direction and come back around looking like something your subconcious would dredge out to haunt you in the middle of the night.
Can you imagine being forced to eat one? I've imagined it, and it's the worst thing I have ever thought of.
Honorable Mention: Anything and everything in Silent Hill
Even in an article full of pus-spewing, razor-toothed horrors, the gore-faced, fleshcrafted abominations that populate the gaming world's most horrifying small town stand head and shoulders (then faceless second head, grafted on mutated arm and topped by a mangled breast that splits open to reveal a melting face) above the rest. Horrifying in that "can't take your eyes off a train wreck" kind of way, these seeping terrors didn't quite make the cut due to the fact that they seems less like an actual "race" and more like random collections of limbs and organs, melted together and shrink-wrapped in discarded skin. However, the debate was kept alive when someone reminded the committee of the single most unsettling cut scene in all of Silent Hill 2, if not the entire history of gaming - the scene in which Pyramid Head forcefully sexes up a pair of "mannequins". We've decided not to classify this as actual procreation, but it was close enough and gross enough to earn the Silent Hill series an special runner-up mention. By the way, the scene is recorded here, in case you forgot. It's quick and it's dark, but also not for the faint of heart.
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