The first time you see a licker in RE2. It quickly scoots past the window as the camera angle suddenly changes to a real tight shot.

Fast Zombies. Now they're all spindly and silly. :)

The Zombies in return to castle wolfenstein. When I was but a wee little Ninja I couldn't even get past the first part of the crypt level out of fear. I have no problems with it now though.

Anarchemitis:
Fast Zombies. Now they're all spindly and silly. :)

Gyeah I still hate those things, they're howls give me goosebumps.

Frank from Donny Darko.

EDIT: Fuck it, man, Frank still scares me.

Mr. Rogers

when I was a kid, I went to the rennaisance fair, and they had this fake medusa head floating in a tank, and it scared the living piss outta me

Buzzy Beetle. I have no idea why, but when I was a kid I had this one recurring nightmare about them at least once a week, and even now I have that same nightmare once every few months, only now it doesn't scare me. I still have no idea why I was so frightened of them.

FLEE IN TERROR!!

Nothing to do with video games but I went to see E.T. in theater at the age of 4. E.T. scared the shit out of me and I had to leave crying hahahaha.

The bosses of Alex Kidd in Miracle World scared the bejeesus out of me when I was a kid. At first it was like 'Paper, Scissors, Rock? Pssh, easy.' Then their heads came off and flew at me. I was terrified.

Anything in the entire game of Daggerfall (elder scrolls.)

The Bloodsuckers and Controllers in S.T.A.L.K.E.R Shadow of Chernobyl were very frightening for me until I started playing S.T.A.L.K.E.R Clear Sky. Everything now can just be easily killed with the auto shotgun you get nearly at the start of the game and the monsters don't even scream anymore.

Like in Halo 3, Flood aren't scary at all. Atmospheric immersion environments are fleeting from sequels.

Monster + Water + Gordon Freeman = Bad

Actually, you can remove Freeman and it still makes sense...

Lvl 64 Klutz:
Buzzy Beetle. I have no idea why, but when I was a kid I had this one recurring nightmare about them at least once a week, and even now I have that same nightmare once every few months, only now it doesn't scare me. I still have no idea why I was so frightened of them.

FLEE IN TERROR!!

Look out, it's coming straight for us!

Flood carrier forms from Halo: CE. Something about the way they just fall over and explode freaks me out.

Anyone remember the third boss from the original Sonic? You know, the one in the Spring Yard Zone where Robotnik didn't have any weapons apart from a Spike in the bottom of his ship that he'd drive into the ground, slowly destroying the platform you were standing on?

He terrified me. This is probably because I'm also scared of heights and you had this looong section where you were carried across a huge drop on a flimsy platform, and Sonic just seemed so fragile at that point... He's actually a really easy boss, now that I look back, but he scared the bejesus out of me at the time.

Another thing that scared me in that game was the random buzz saws that would appear in the Scrap Yard Zone. Those things would come out of NOWHERE to get you. Then there's also the classic *Sonic's about to drown* noise which panicked me beyond belief...

Wow, on recollection the first Sonic game really did a number on me. Do bear in mind that I was 6 at the time.

Not from a game, but I was terrified of It of the eponymous movie. I only saw one scene, the one where It talks to Georgie, but it did more to me than anything else.
Now I shoot clowns in the head, just in case. I did conquer my fear, but it's more of a safety measure than a fear ;)

Sharks still freak me out. I once read MEG by...Stephen Alton or something...and after I finished it, I went to row a boat (with my parents) on a big body of water. In the book, the Megashark is in a lake at one point. It scared my tits off. Then I realised that it was a mountain lake and nothing could get there. Until I realised that it might have been there before it was a mountain lake. Being a writer myself, that got me going real bad.

Just imagine being in a black space with nothing around you. And then, a giant whale or shark swims at you and gobbles you up with one bite. Writing this already gives me goosebumps. >.>

Velociraptors
those fire throwing whatchamacallits from Doom

I used to be terrified of the eels from Super Mario 64. That all ended after the epic showdown with a massive eel in Super Mario: Sunshine.

edit: actually it ended a little before that...

the undead, mostly mummies and zombies with Frankenstein making the occasional guest appearance outside my door...monsters never lived in my closet

Morrowind's Corprus creatures.

Seein as though everyone has stolen my Zelda ones ill talk about somthing completly off topic.

When i was 6 or 7 i went to movie world for the first time when the gremlin ride was still there and not the scooby doo coaster. Anyway we went on the gremlin ride and it scared the shit out of me for months then as soon as i was getting over it i watched the movie and my fear resparked. I slept with the light on for so long cus i knew they didnt like light.

now of course i love gremlins and its one of my favourite movies. Irony is such a bitch.

The aliens from Alien. Those things scared the shit out of me.
But I'm over it now, so it's all good.

AgentCLXXXIII:
The snowman in Ski Free.

You mean robot?

Fast headcrab Zombies. Or, actually, just Ravenholm in general. I was out of ammo, and had to fight off 3 with the crowbar. And then the flashlight ran out.

The flood as well. Getting lost in a complex with infinitely spawning parasitic octupi and zombies is NOT fun.

Easily Bowser's clown copter thing in Super Mario World, and
more easily the redead in Zelda: Ocarina of Time. I still get the chills from those things.

kanada514:
Nothing to do with video games but I went to see E.T. in theater at the age of 4. E.T. scared the shit out of me and I had to leave crying hahahaha.

I recall bringing that movie to my preschool to watch and nearly all 30+ of the kids cried and left the room when ET came on screen except me and like 2 other kids. I guess ET is - with those formative years behind me - really just a benevolent sausage monster with a heart of gold.

Well... When I was REALLY little, we're talking one of my first ever games...

You see... <.<...>.>... *Pre-digs his "bury-head-in-embarresment-here hole"*

The Grasshoppers from A Bug's Life.

Here me out. It's level two, I'm walking through the caves, been avoiding those red worm things, about to go get that grai- OHMYFECKINGGODITJUSTCOMESOUTOFTHECEILINGMYWEAPONDOESNOTHINGAAHHH!!!
*Shudder*

Oh and video games? The bad guy from police quest 3 scared me. He was a satanic serial killer who stabbed your pregnant wife. Kinda heady stuff for a 10-year old.

movies:
Critters

'nuff said

The moomin trolls

Dont laugh.....

Ravenholm *shudder*
I openly admit that Fast Zombies scare the shit out of me, but I for one can't stand the screams of the normal ones when you set them on fire, it just creeps me out so much.

Listen for yourself

Dude, the Blob. That thing scared the SHIT out of me, especially the part where it fell on the guy and ate him like acid.

And the GET OUT sound from Donkey Kong 64. Not a monster, but really really wetyourpants scary.

Since PH was already mentioned....

The Split Lizard from SH1
The final boss of SH3
The big attached to the wall thing in the final battle at SH4
The Fog and darkness from pretty much every SH game.

If you consider the whole town as a monster, then include Bleeding walls, rusty metal gratings and static noise to that list.