In Forgotten Realms, the Simbul used to throw Red Wizards into mindless panic just by appearing. Mostly because of her bad habit of killing them on sight. That, and being probably the most powerful magic user on Toril. (Some descriptions state that as far as level and abilities go, she's even stronger than Elminster.) Others don't run for cover, but still instantly sober up at a mention that she may or may not be involved — like Cormyreans did in All Shadows Fled.
Pick a name from Warhammer 40,000 and you'll find a few billion people (and xenos), at least, shitting their pants in fear from the mere thought of them. Maybe a few 'Crons, a few 'Nids, some Dark Eldar or someone straight from the Eye of Terror, and even heroes of great renown find that they have to draw on every ounce of their resolve not to run away or die.
Special mention to the Night Lords. Entire solar systems have surrendered rather than battle them (tip: this doesn't stop them from butchering you).
Kharn the Betrayer gets points for being so Axe Crazy that even other Khorne Berzerkers have been known to freak out when he shows up. It's not just that he's an unstoppable murder machine, he's an erratic, teamkilling unstoppable murder machine.
Within the Imperium, Inquisitors. Not just because they're personally Badass, although many are, but because they have effectively unlimited power. They can order your whole planet sterilised or just take you away to torture, and no one will, officially, say anything. Just the sight of their badge of office has been known to make people soil themselves.
The Necron Pariahs have this as their entire hat. In a certain area around them, almost any creature experiences crushing, debilitating dread. In a Ciaphas Cain novel, the crack Stormtrooper squad that had been shown to be far and away more cohesive, ruthless, and cunning than any Guard squad Cain had ever fought with falls completely apart in the presence of Necron Pariahs, gibbering and crying and so forth while the Pariahs casually slice them apart.
The Eldar Harlequins. To put it into perspective, the Dark Eldar (who probably count as an example of this trope as well) are a race of murderously psychotic beings with an almost genetic urge to rape and torture people in horrific ways, are utterly terrified of the Harlequins and dare not refuse a Harlequin troupe access to their normally impregnable inter-dimensional city, even if they're just there to put on one of their shows. Monster Clown, taken Up to Eleven.
Warhammer, the fantasy-themed sister universe of the above, isn't lacking for these either.
Archaon, the overlord of Chaos. The warriors of Chaos stand out as being an entire race of The Dreaded, and this guy is the biggest, baddest of the lot. He has all four of the Chaos Gods backing him, and he has almost destroyed the world several times.
Malagor, most powerful and infamous of the Beastman shamans. Also known as the Dark Omen, the Crowfather, the Despoiler of the Sacred, and the Harbinger of Disaster. So feared is he that the Cult of Sigmar in the human Empire vilifies him as the epitome of sin and blasphemy. He actually has an in-game special rule that prevents enemy units from using their general's Leadership; in other words, Malagor scares the other races so much that their own leaders can't keep their troops in line unless they're right next to their troops.
Also from the Beastmen, Gorthor. Even the Beastmen themselves were shit-their-pants terrifed of him, as evidenced by his name being Bray-tongue for "Cruel". You want to know why? Many Beastmen wear human skins as a matter of course. Gorthor wore the skins of Beastman shamans. Most Beastman wouldn't dare to touch a shaman, and this guy went around wearing their skins. As for how the humans view him, well, they talk about him the way they talk about Archaon. Archaon is still alive. Gorthor died one thousand years ago.
The Ogre Bragg the Gutsmen is this to his own people, because he's the only Ogre who dares to disembowel his rivals, which is, to Ogres, the most cruel, disgusting and horrific way to die imaginable. Thankfully, many argue that Bragg couldn't become Tyrant in his own right because so many Ogres are too scared to stick around and obey him.
With a name like Nagash the Undying, you're bound to be this. He's the inventor of Necromancy. The living humans of Nehekhara refuse blankly to speak his name, and the only time in history the Skaven united was to bring him down, and when they failed, the entire empire stayed away from him.
Mentioning Shadowblade in a room full of Elves is a good way to make them all go deathly quiet. He's the greatest assassin the Dark Elves have ever produced, and more importantly, their best infiltrator. (Most assassins can hide in their army's forces; Shadowblade can hide in the enemy army.) He's called Shadowblade because he personally killed everyone who ever knew his real name, save maybe for Malkeith, and that's only because he's on Malkeith's payroll.
Anyone in the Dungeons & Dragons universe who isn't at least level 20 will metaphorically and/or physically crap their pants at the news the the Tarrasque has awoken. The beast, while occasionally fading into myth due to its long hibernation periods, is NEVER totally forgotten, as when it IS awake, it embarks on rampages of destruction and feeding that can destroy whole countries. Even if it is defeated, it requires the use of the top-level arcane spell Wish to keep it from just brushing off death. Only the most powerful spell in the game can kill it.
Out-of-character, rust monsters have a fearsome reputation, for their ability to eat your weapons and armor, leaving you defenseless — the iconic "screw you" monster. Similarly, the Adamantine Horror, which can destroy even powerful magical items with at-will disjunction. And it can also use disintegrate at will, another incredibly powerful spell. (Keep in mind, this monster is Challenge Rating 10 in the 2nd Edition, meaning it's supposed to be the equivalent of level 10, many levels below any PC can gain access to either spell).
On a meta level: Dragons and even the Tarrasque may be met with a cry of Charge! from all around the table, but throw a vampire at the party, and it's "run away, those things can Level Drain!"
Vecna the Lich God of Secrets is so feared that few mortals even say his name aloud; instead, most use his titles, such as The Maimed Lord, the Whispered One, and the One Spoken Only In Whispers. Even when he was mortal, he was so notorious for his cruelty that few dared say his name, most referring to him by his original title, the Master of the Spider Throne.
3rd Edition's sister product, d20 Modern has the Frightful Presence Feat, which requires hostiles to make Will Saves or become afraid just by seeing the character who has it. To be fair, enemies of a higher level are unaffected. The same ability appears in the 3rd Edition Dungeons & Dragons, but is drastically more difficult to obtain.
The Lady of Pain is this to anyone who lives in Sigil in the Planescape campaign, and with good reason. It takes a lot to make her angry at you, but if you do make her mad, she locks you up in an eternal maze. (And that's if she's in a good mood. If someone catches her on a bad day when they make her mad, then magic is going to be needed to identify the body.)
The Old World of Darkness had beings known as Antedelluvians. So named because they were first made vampires before the Great Flood. Being third generation, meaning their sires were embraced by Caine himself (who falls under this trope as well), they are ungodly powerful. So powerful in fact that just ONE waking up on the Indian Subcontinent required forces from several factions teaming up, and magickally enhanced nuclear weapons. All of which only WEAKENED it enough for orbital mirrors concentrating sunlight on it to finish it off.
Don't forget, Antedelluvians have access to tenth level vampiric disciplines. The most common ability at tenth level? "Plot Device".