I have many but I’ll stick to HL. The HL2:Episode 2 Guardian that you couldn’t kill plus I’m not sure if this could be considered a monster but in HL1 there were those screams that played, you may know them as the sound files “thehorror” or damned souls from Freeman’s Mind. I’d assume those were some sort of creature, not human but not a creature you ever encounter.
It was the sound of Mr. Friendly.
Well, his victims, anyway.
My first outdoor bloodsucker experience was in that wild region by the Bar… on the railway platform, something got behind me and went to town, and it was still nearly as terrifying… and I’m pretty sure there’s 1 other vanilla location where they see sunlight: that vacant village west of Freedom’s warehouses which had a quest to wipe out the ones swarming it.
Hahaha… When I heard that thing moping around, I went to plan B: GRENADES GRENADES GRENAAAADES!!!
Grenades?
Pfft, I always knifed the motherfu… eh, ran away from him.
Seriously, the best way was not to waste the ammunition.
Just what I thought when he said “a monster you will never encounter”.
Brutes.
'Nuff said.
Care to specify?
He’s probably talking about Amnesia
The monsters from Amnesia. I think i shit myself at one point.
That’s really always the first monster I think of of when something like that comes up. Mr. Friendly is also the first monster of think of when I think “Beta Half-life” Although in this case that wasn’t the first thing to come my mind but I knew someone would say it.
Those are pretty cool. I love the S.T.A.L.K.E.R. games.
But what I really like are the Pinky demons in the original Doom.
I’m obliged to put my favorite monster considering.
One tough badass.
Also William Birkin from Resident Evil 2
Those two are way to overdone to be scary
AAUUUUUUUUUUUUUUUUURROOOOOOOOOOOAAAUUUUUUUUUUUGHHH
I can’t remember what it’s called, but the monster in the factory in Call of Cthulhu was pretty bad.
Worst ones for me were the monster in FEAR that come up out of the ground. I don’t like stuff appearing underwater anyway, so in one ‘scare scene’ where one tries to grab you in a hospital corridor filled with blood, it freaked me out and I was scared of the water for the rest of the game. Only got attacked in the same way once, partway through the first expansion in some cave bit, where I was too scared to react quick enough to kill the monster. 
Clowns – I HATE CLOWNS. And not just in games. I’ve mentioned this before, the witches in L4D2 really creep me out. I manage to avoid them as much as possible. I must say, I have a pretty good record as far as killing them with a shot gun or chainsaw. Not so much on crowning them though. :[
Two NPCs I usually think of when I hear the beta version for…
Half Life 1 - mr. friendly
Half Life 2 - the cremator
I think the sirens from amnesia and basically everything from S.T.A.L.K.E.R is a sure winner in this thread.
grunts :’(
Pussy.