The creatures in Alien are pretty scary. The idea of something getting inside you and gestating, feeding off your internal organs, until it bursts out of your stomach while you're still alive...
The earwig things from Wrath of Khan are freaky nasty, too - crawl in your ear and eat through your brain, causing agonizing pain, insanity, paralysis and death...
Several years ago I woke up one morning, took a shower to get ready for work and as I was getting out of the shower, noticed my ear was stopped up, like I'd gotten water in it, but I was pretty sure I hadn't. I shook my head a few times, but couldn't dislodge it, so I went on to work.
Mid-morning, I'm working at my computer, when I notice the water in my ear hasn't evaporated yet, so I reach up and rub my ear real hard.
And something inside my ear moves. Not just shifts, but wriggles around!! GaAaAaaAAaaAH!! !
Take my lunch break, go to the nearest WalMart in a state of panic - what kind of medicine do they make that gets living creatures out of your ears? NONE. There is, however, an ear mite medication for dogs. It'll have to do, I can't just ignore it and wait for it to go away.
So I go back to the kitchenette at work, and using a rubber bulb syringe (like you use to rinse out a baby's ears), I fill the bulb with the ear mite medicine and water running from the tap and squirt the mixture in my ear. Simultaneously, the creature BITES ME ON THE EARDRUM and I realize the only water that comes from the kitchenette tap is HOT WATER!! !
I tilt my head into the sink and out washes one of those fuzzy little black house spiders, who apparently had just crawled across my pillow as I slept and made him/herself at home in my head.
Later I tell this story to my 93 year old grandmother who grew up on a farm where experiences like this were not so freakishly unusual and she says: "You could have just washed him out with cold water, he'd come out rather than get drowned." DUH.
The scariest monsters are the real ones.
Sleep tight. 