The most fearsome, frightening monster in any RPG is...

[United Nations]

SOMA:

 

[oldmanpaco]

These fuckers:

Every time I turned a corner and saw these guys hanging from the ceiling I'd jump. If they were on the ground - no problem, just the ceiling ones started me for some reason.

 

[Ippolit]

Wow, that's beautiful; where did you find it? My google-skills are inferior.

I will never stop getting nervous against these 2 enemies:

And this is from Lufia 2, blue mimics. I was so happy when i found these the first time playing as a kid. Full wipe ofc. Today not particulary scary but durable and if they are positioned badly, they can ruin your day in the first runs.

But for any RPG...due to my limited experience i'd vote for this one:

 

[makiavelli747]

 

[POOPERSCOOPER]

Your Momma.

 

[Mustawd]

Fuck u..Chrysalids...Fuck you in the asshole!!

 

[Grotesque]

 

[Wayward Son]

Have to add these, as I only recently found them (read: past few days):
Sword Spiders in BG, fuckers attack like they're hasted and deal damage to boot.
Battle Horrors near Durlag's Tower. Finding them at level 3-4 will haunt my nightmares. Im the kind of person that has a hard time saying, "No, I'll wait and get them later." So I spent several hours trying to beat them and finally did for 4K xp a pop.
EDIT: ^ Yep. I feel like that'll be me in fifteen years.

 

[Dedup]

Daedra Lords in Daggerfall. You would be in a dungeon at low level, not having many issues as you search for what ever quest item/enemy you have to find and you hear that fucker's laugh. Now you're on edge hoping you can finish the quest and get the hell out before running into him because you know you have no shot of beating him.

 

[Doktor Best]

He was a cutie when he was a baby though:

 

[Zed Duke of Banville]

Wow, that's beautiful; where did you find it?

http://pixeljoint.com/pixelart/103722.htm

Pixel Joint has some beautiful art mixed in with a vast amount of mediocre material.

Spoiler

 

[Aothan]

always found Beholders in any moderately challenging and balanced NWN persistent world to best be avoided, others had different ideas

 

[Lady_Error]

Not a monster as such, but going through Ravenholm in Half Life 2 was pretty much like being in a horror movie.

 

[octavius]

The monsters I most dread facing when playing low level parties in FRUA:
Drow
Bulette

 

[Zetor]

The ghosts in the White Tower (level 3), Lands of Lore. Very high - potentially party-destroying - damage, immunity to normal weapons and spells, and the fuckers WALK THROUGH WALLS. That entire level was basically nightmare fuel for my 12-year-old self.

(Yeah, they become a lot less scary if you read a walkthrough and know that they're vulnerable to a certain weapon as well as a certain magical item if you exploited its properties earlier... and they don't respawn. But that's all metagaming, yo!)

 

[Jack Of Owls]

The mimics in the Neverwinter Nights module-remake of Eye of the Beholder. They were only found in a huge, sprawling and deadly secret level. They looked like a perfectly ordinary treasure chest, until you tried to open it, thus playing right into EVERY adventurer's primal instinct and deadly sin - greed.. Then they would turn into-damage personified with great, chomping teeth coming right at you like a biological buzzsaw from a lost circle of Hell, pursuing you. The Terminator himself would have run from these bastards. They were enormously tough to kill, and once you did, they would explode in a splattering, lethal spray of glowing green acid or orange fire. Even a ranged attack would not spare you from the spray. The mimics make appearances in other NWN modules, and the Japanese game developers were so impressed by them that they made a memorable appearance in the Dark Souls series. But in the EotB module, they were much more fearsome. Aside from the rust monsters and the beholder boss at the end, they were the only enemies I truly feared in EotB.

 

[Xathrodox86]

Ming Xiao comes to mind.

 

[k0syak]

In Dark Sun 2: getting hyped for a final showdown with the Dragon (not some pussy FR dragon, mind you) and then facing this

Spoiler

Spoiler

Tarrasque

Climate/Terrain: Any land
Frequency: Unique
Organization: Solitary
Activity Cycle: See below
Diet: Omnivore
Intelligence: Animal (1)
Treasure: See below
Alignment: Nil
No. Appearing: 1
Armor Class: -3
Movement: 9, Rush 15
Hit Dice: 300 hp (approx. 70 HD)
THAC0: -5
No. of Attacks: 6
Damage/Attack: 1-12/1-12/2-24/5-50/1-10/1-10
Special Attacks: Sharpness bite, terror
Special Defenses: See below
Magic Resistance: Nil
Size: G (50' long)
Morale: Champion (15)
XP Value: 107,000

The legendary tarrasque, for there is fortunately only one known to exist, is the most dreaded monster native to the Prime Material plane. The creature is a scaly biped with two horns on its head, a lashing tail, and a reflective carapace.

Combat: The tarrasque is a killing machine and when active (see below) eats everything for miles around, including all animals and vegetation. Normal attacks are with its two forelimb claws (1d12 points of damage each), a sweeping tail lash (2d12 points of damage), a savage bite (5d10 points of damage plus acts as a sword of sharpness, severing a limb on a natural attack roll of 18 or better), and two thrusting horn attacks (1d10 points of damage each).
Once every turn, the normally slow-moving tarrasque can rush forward at a movement rate of 15, making all horn attacks cause double damage and trampling anything underfoot for 4d10 points of crushing damage.
The mere sight of the tarrasque causes creatures with less than 3 levels or Hit Dice to be paralyzed with fright (no saving throw) until it is out of their vision. Creatures of 3 or more levels or Hit Dice flee in panic, although those of 7 or more levels or Hit Dice that manage to succeed with a saving throw vs. paralyzation are not affected (though they often still decide to run away).
The tarrasque's carapace is exceptionally tough and highly reflective. Bolts and rays such as lightning bolts, cones of cold, and even magic missiles are useless against it. The reflection is such that 1 in 6 of these attacks actually bounces directly back at the caster (affecting him normally), while the rest bounce off harmlessly to the sides and into the air.
The tarrasque is also immune to all heat and fire, and it regenerates lost hit points at a rate of 1 hit point per round. Only enchanted weapons (+1 or better) have any hope of harming the tarrasque. The Tarrasque is totally immune to all psionics.

Habitat/Society: It is fortunate that the tarrasque is active only for short periods of time. Typically, the monster comes forth to forage for a week or two, ravaging the countryside for miles around. The tarrasque then seeks a hidden lair underground and lies dormant, sleeping for 5d4 months before coming forth again. Once every decade or so, the monster is particularly active, staying awake for several months. Thereafter its period of dormancy is 4d4 years unless disturbed. The ratio of active to dormant states seems to be about 1:30.

Ecology: Slaying of the tarrasque is said to be possible only if the monster is reduced to -30 or fewer hit points and a wish is then used. Otherwise, even the slightest piece of the tarrasque can regenerate and restore the monster completely. Legend says that a great treasure can be extracted from the tarrasque's carapace. The upper portion, treated with acid and then heated in a furnace, is thought to yield gems (10d10 diamonds of 1,000 gp base value each). The underbelly material, mixed with the creature's blood and adamantite, is said to produce a metal that can be forged by master dwarven blacksmiths into 1d4 shields of +5 enchantment. It takes two years to manufacture each shield, and the dwarves aren't likely to do it for free.
It is hoped that the tarrasque is a solitary creation, some hideous abomination unleashed by the dark arts or by elder, forgotten gods to punish all of nature. The elemental nature of the tarrasque leads the few living tarrasque experts to speculate that the elemental princes of evil have something to do with its existence. In any case, the location of the tarrasque remains a mystery, as it rarely leaves witnesses in its wake, and nature quickly grows over all remnants of its presence. It is rumored that the tarrasque is responsible for the extinction of one ancient civilization, for the records of their last days spoke of a "great reptilian punisher sent by the gods to end the world."

Note: Creatures with a minus THAC0 can only be hit on a 1.

It did go reasonably well, though

Spoiler

nothing beats STR24 half-giants dual-wielding El's Drinkers

 

[Dux]

Being chased by Orcs in Gothic I/II is quite nerve-wracking at times - especially when they run faster than you.

I remember playing as a squishy mage in Gothic II, trying to goad one Orc away from the pack only to aggro the whole bunch. So I swivel around and run like hell, listening as their guttural cries grows louder at my back.

 

[Orobis]

Monkeys and midwives...

 

[Outlander]

Fuck that shit.

 

[Wintermute]

Armor Class: -3
THAC0: -5
...
Note: Creatures with a minus THAC0 can only be hit on a 1.

Nope. It only misses on a 1 (if your AC >= -5, or actually -7 as everything higher is hit at 2+). You can hit it at your THACO + 3 as it's AC is -3.
The lost art of THACO, not even the Monster Manual (that is the source here, isnt it?) can get it right.

 

[k0syak]

Nope. It only misses on a 1 (if your AC >= -5, or actually -7 as everything higher is hit at 2+). You can hit it at your THACO + 3 as it's AC is -3.

Affirmative

The lost art of THACO, not even the Monster Manual (that is the source here, isnt it?) can get it right.

Affirmative

 

[k0syak]

Being chased by Orcs in Gothic I/II is quite nerve-wracking at times - especially when they run faster than you.

I remember playing as a squishy mage in Gothic II, trying to goad one Orc away from the pack only to aggro the whole bunch. So I swivel around and run like hell, listening as their guttural cries grows louder at my back.

Drink haste potion --> cast storm --> run through orcs --> run back to cover --> repeat.
Also, in the colony, you can gather about two dozens of them (plus wargs) in front of the battering ram and nuke away.

 

[Saxon1974]

I think its why no saving except in towns was good. In bards tale or might and magic when you were deep in a dungeon and had run out of magic and potions you got nervous! I know modern games will not do that. The save/reload mechanic kills tension