Monsters have been a part of human culture since we invented the very concept of stories. They've been warnings. They've been metaphors. They've been exaggerations of reality. In some way or another, we're all scared of monsters.

And monsters are especially scary when they're not from our world.

With 'Riddick' now in theaters, it's time to look back at the alien movie monsters that have plagued our dreams the most. The creatures from distant planets who want nothing more than to tear us to pieces, eat our brains or force us to join to their disgusting empire. When Riddick is done killing his way through an army of aliens in his own movie, these are the 10 we'd like for him to take on next.

A special note: we took the phrase "alien monsters" very literally here. We have deliberately avoided movie aliens who showcase extreme intelligence or have a deep understanding of technology. Our focus here is on the extraterrestrial equivalent of wild animals, acting on instinct and often little more. These are alien monsters, not aliens. We bend this rule on occasion, but this is why you won't find something like the Predator on this list.

  • 10

    The Rancor

    To the adult mind, the Rancor isn't too scary. But do yourself a favor and flashback to the first time you saw 'Return of the Jedi.' Remember Luke Skywalker being thrown into the pit underneath Jabba's palace. Remember the massive beast emerging from the shadows, thick dribble spilling out of its maw. Remember it chowing down on that unlucky Gamorrean guard in what may be the most unnecessarily brutal moment in the 'Star Wars' saga. We've seen alien movie monsters do so much worse since we've grown older, but for an entire generation, the Rancor is pure nightmare fuel. The 'Star Wars' films never got too brutal and were rarely scary, but then George Lucas decided to open one with Luke battling a creature that looks like it escaped from the bowels of Hell. Thanks for the bad dreams, George!

  • 9

    The Color Out of Space

    The alien entity lurking at the heart of the crashed meteor in the 1965 cult classic 'Die Monster Die!' is never given a name, but since the screenplay is based on H.P. Lovecraft's chilling 'The Color Out of Space,' that is what we shall dub it. What else could we call it? This thing is barely a creature -- it's a sentient and malevolent shade of color from beyond the stars. It's like radiation, but it has the urge to hunt, kill, ruin and mutate. Naturally, the wealthy family at the center of the film try to use this power for their own ends, resulting in mutant plants, mutant animals and a mutant Boris Karloff. You should never trust meteorites that land on your property on general principal, but you should be especially wary when they emit a green light and seem to be driving everyone crazy.

  • 8

    The Arachnids

    Paul Verhoeven's 'Starship Troopers' is a deliberately silly B-movie send-up filled with intentionally campy dialogue, overwrought story twists and biting satire. But it's also terrifying as all get-out. As funny as the movie is, the alien "Arachnids" that our heros battle throughout the film are no laughing matter. From the seemingly infinite soldiers to the massive plasma-shooting beetles to the mind-devouring "Brain Bugs," these beasties look and act like they came from a completely serious sci-fi movie. Human characters are literally torn to shreds by these guys throughout the film, with each battle sequence new and disgusting ways to dismember a human. The movie may be a big joke, but the bad guys? They're no laughing matter. And that may be the biggest gag of them all.

  • 7

    The Pod People

    There are few things as unsettling as human beings who have lost their humanity. In both the original 1956 'Invasion of the Body Snatchers' and it's 1978 remake, alien "pod people" arrive on Earth and begin to assimilate themselves into society, killing off ordinary people and replacing them with soulless clones. The result are monsters that are truly terrifying: they're ourselves minus everything that make us who we are. It's an alien life-form using our humanity as a mask to grow its numbers. It's the decimation of our culture and our way of life, right under our noses.

    Oh, and there's also that TERRIFYING MUTANT DOG from the remake that will haunt our dreams forever.

  • 6

    The Id Monster

    'Forbidden Planet' is one of the best science fiction films of the 1950s, but it's rarely remembered as being scary. Watch it again. Although the bulk of the film is a thoughtful and occasionally goofy sci-fi tale, the monster that threatens the crew of the United Planets Cruiser C57-D is one of the cleverest and freakiest villains in movie history. After picking off people one-by-one without being seen, the crew eventually realizes that their adversary on the planet of Altair IV is invisible. This leads to the jaw-dropping scene where they use a laser fence to illuminate it, revealing an animated monster that looks like Satan designed by Walt Disney. When combined with the film's last-minute revelations about the creatures, this image is the kind of thing that gets burned into your brain and just stays there. Show it to you kids. Ruin their sleep.

  • 5

    Big Alien Gorilla Wolf Motherf---ers

    The aliens at the center of the wonderful 'Attack the Block' are never given names, but the London street youths who battle them don't hesitate to give them a variety of colorful nicknames (like the one we chose for this list). Unnamed or not, these creatures sport one of the cleverest monster designs in recent cinematic memory. Their shaggy fur is so black that it seems to absorb light, so the only way to see them coming is to look for their teeth, which glow an eerie green. The result is a monster that looks like a shadow with teeth. Well, a shadow with teeth that's the size of a bear and has the ferocity to match. In an age where most aliens are being completely and needlessly over-designed (see: 'Super 8'), these guys looks like something torn from a young child's nightmare. They're scary because they're so simple.

    Screen Gems
  • 4

    The Blob

    In both the 1958 and 1988 versions of 'The Blob,' the title alien is essentially the same: a gooey mass of something that devours anything with a pulse, constantly growing and constantly moving, searching mindlessly for new prey. Although the remake is significantly more violent, both takes are equally frightening in their implications. The Blob doesn't have a mind of its own and it doesn't have a grand plan. It's an entirely unknowable alien organism, operating solely on instinct. Most movie monsters have a head or eyes or a mouth, something to remind us that they're a living being that can be killed. But not the Blob. You stare at it and it doesn't stare back. You can't placate, distract or escape from something that knows eating and growing and killing and nothing else.

  • 3

    The Thing

    What makes the alien being at the center of John Carpenter's 'The Thing' so terrifying is that it doesn't have a recognizable form. You won't see it coming. You can barely prepare to fight it. More of a virus than a monster, it's a shapeshifting being that can take on the form of anything it comes into contact with. Every cell has a mind of its own, so it can split into multiple forms, living on and thriving as long as at least one tiny piece of it remains alive. Although the Thing itself can take on whatever disgusting form it desires, the true fear comes from the fact that it could already be anyone around you. Sure, it can tear you limb from limb, but it's true secret weapon is the ability to sow distrust and anarchy, turning human against human while it mops up the remains.

  • 2

    The Killer Klowns

    The Killer Klowns (as seen in 'Killer Klowns From Outer Space') are probably a bit more intelligent than most of the extraterrestrial monsters on this list. After all, they have their own spaceship and seem capable of galactic travel. However, what makes the title creatures in this gloriously campy cult favorite so terrifying is that every single thing about them feels built to be unnerving. These are simply terrifying alien creatures that look exactly like clowns and hunt humans. They don't appear to have a grand plan or a motive that makes sense -- they're just pure nightmare fuel, a bad dream come to absurd and grotesque life. There are scarier films on this list, but few movie monsters are as downright uncomfortable as these guys.

  • 1

    The Xenomorph

    Could anything else have even come close to the number one spot on this list? It's not an opinion that the Xenomorphs are the most terrifying aliens in cinematic history, it's a stone cold scientific fact backed up great reams of evidence. You'd think the popularity of 'Alien' franchise would have dulled these nasty beasts' impact by now, but if anything, they've only gotten more frightening with age. Although James Cameron's 'Aliens' uses an army of these guys to nerve-shattering effect, they've never been more terrifying than the lone creature hunting the crew of the Nostromo in Ridley Scott's 'Alien.' Rarely seen in full view and constantly changing its form, this acid-blooded monstrosity is the stuff of nightmares, a perfect killing machine that is so far removed from everything we can comprehend that it's truly, well, alien.

    20th Century Fox