Angel: Has The Destroyer, a very powerful creature transfer over from the hell dimension Quor'toth, who is revealed to be Angel's son Connor.
Breaking Bad: After the murder of Gus Fring, Walter White, by the name Heisenberg, is known and feared throughout the American southwest.
Buffy the Vampire Slayer: The big bads tend to be this to the heroes and usually for a really good reason. With the exception of Angelus in season 2 (who was himself quite strong and had an almost foolproof plan) all of the villains have been nigh unstoppable, cunning and highly destructive. The full list is an ancient vampire, three mad vampires (with a portal to hell), a pure demon, a cybernetically enhanced demon (made from a handful of demon corpses), a mad god trapped in a human body, a witch capable of killing gods and the source of all evil.
Burn Notice: The main character develops this kind of reputation over the course of his spy work before the series starts. One agent admits his agency always assumed "Michael Weston" was just a code name for a group.
The Cape: Chess is this, the mere mention that he could possibly be alive being enough to spook public defender Dana Faraday.
Cheers: Norm became this in one episode, when the CEO of his company forced him to become the company's hatchet-man, essentially giving employees notice that they had been laid off. At the end of the episode he decided he could not keep this job with a clean conscience, so he decided to turn in his resignation. He tried calling his immediate supervisor and then the CEO's secretary, both of whom hung up immediately; word had gotten out through the company grapevine that if you got a call from Norm Peterson, it meant you were being laid off. When he realized what was going on, he called up the CEO personally and greeted him with, "Hello, sir? This is NORM!PETERSON!MUAA-HA-HA-HA-HA-HA!"
Dexter: When one of George King AKA 'The Skinner''s employees is brought in for questioning about a series of murders, he has a near panic-attack, and would rather go to jail than convict him, and for good reason.
Doctor Who: The Daleks are treated by all who know them as the bogeymen of the Doctor Who universe. Even the Doctor, who's battled and defeated them uncountable times, always treats their arrival with a mixture of grim determination and just plain old fear. Best seen in the end of the first part of "The Stolen Earth". When the first broadcasts of "EX-TERM-IN-ATE! EX-TERM-IN-ATE!" reach Earth, the former companions and allies of the Doctor such as Sarah Jane, Martha Jones, and even Jack Harkness — some of the bravest people in the universe, one of whom is outright impossible to kill — are stunned into a state of tearful panic.
When the Daleks are first encountered in the new series (in the eponymous episode Dalek), the Doctor reacts with sheer panic when he realizes the alien prisoner he is locked in with is (apparently) the last surviving Dalek.
After the reboot of the universe, Rory Williams has been interwoven into the myths and legends of human history as "The Last Centurion". Woe betide anyone who threatens his loved ones;
Should be noted, before certain events in Series 7, The Doctor qualified for this trope. In that his name and history was enough to terrify plenty of alien races. Including, irony abounds, the Daleks. The only thing that gives a Dalek pause is the Doctor's name. The only thing that scares them is a madman with a box, the one presence in the cosmos they would not wish to face.
Even the Doctor's regenerations face this. There is a regeneration those following the Eighth Doctor don't speak of: the War Doctor. He is notorious for breaking the promise bound to the name "the Doctor" by burning down Gallifrey and the Daleks with them. note Of course then it's found out in the 50th Anniversary special that Gallifrey didn't burn and was instead preserved in stasis by all of the Doctors (It Makes Sense in Context), and the only reason the timeline still asserts the War Doctor as The Dreaded is because he and Ten forget they actually saved Gallifrey after they return to their own times.
Just the mention of River Song's name is enough to make a Dalek literally beg for mercy. It is not granted.
In The Day of The Doctor we are treated to two back to back encounters of The Dreaded: first, as a group of Daleks invade Arcadia, the second city of Gallifrey, a swarm of the Omnicidal Maniacs surrounding a group of unarmed, defenseless Gallifreyans, including scared, crying children, and then we get the second, as, before the Daleks can slaughter these people, they all detect the presence of The Doctor, and dare not waste even a minute fraction of their attention on anything but their long-hated foe, with not even one staying behind to finish off the helpless people.
Family Feud: During the Richard Dawson era, he would often preface a particularly bad answer with the words, "The dreaded … ," always drawing audience laughter. A rare game show example, this was always meant to be taken in jest.
Grimm: The Grimms are The Dreaded of the Wesen world. Whenever someone realizes Nick is a Grimm for the first time, they are terrified and often beg not to be killed. Monroe explains it best;
Monroe: You're the monster under the bed! [...] You're not real! You're a scary story we tell our kids! Be good or a Grimm will come and cut your head off...
And like Harry Dresden or the Doctor, he's beginning to earn a reputation for himself one encounter at a time as people get wind of what he went up against and what the outcome was. He'd deny being one of the baddest badasses out there, but the scorecard says he is.
Heroes: Sylar. When a gaggle of supervillains are released from their cells and have the chance to take revenge on Noah Bennet, the guy who locked them up, Noah only has to say one thing to scare them into teaming up with him: "Sylar's in the building."
Bennet was pretty dreaded himself at first. He's a Badass Normal who used to work for The Company and was very good at it. Sylar is in fact the only thing more feared than "the man with the horn-rimmed glasses."
Human Target: Guerrero has managed to interrogate people just by introducing himself. Of course, being prime time they have to keep most of the stuff that got him the reputation off screen.
Kamen Rider Kiva: Wataru develops into this after recovering from his Heroic BSOD, as demonstrated where he begins sends Fangires fleeing in terror simply by staring them down. Without transforming into Kiva.
Merlin: Merlin's older alter-ego has this reputation towards Morgana in series four. While a heroic example, he's not a Terror Hero as this is completely unintentional on his part, and she's terrified because the gatekeeper of the spirit world told her that he is her destiny and her doom. Whenever she sees him in his aged form, she has Tears of Fear. It doesn't help that he commanded a dragon to burn her army, stripped her of her powers once, and in a magical battle, completely curbstomped her and tossed her aside like a rag doll.
Morgana's vision of the future plays this straight. She's lying wounded on a battlefield, littered with the remains of her dead soldiers. And then we see the older Merlin, standing over her triumphantly and seemingly all-powerful, furiously admonishing her for all of her crimes.
Merlin: Is this really what you wanted, Morgana!
Monty Python's Flying Circus: Everyone in London is terrified of Doug Piranha, including his brother and partner in crime Dinsdale. "I've seen grown men pull their own heads off rather than see Doug," Luigi Vercotti says. Doug's most horrifying technique? Sarcasm.
Twin Peaks: Bob. An evil spirit that raped and murder Laura Palmer. Possesed Leland and murdered Maddi.
Season 10: Smallville's version of Darkseid. In fact, that's the whole point of him: as long as you have fear or doubt in your soul he can control you.
Spartacus: Blood and Sand: Theokoles, the Shadow of Death. This giant, undefeated gladiator terrifies everybody when he's announced to be coming out of retirement. Everyone assumes that Spartacus and Crixus are doomed when they are announced as his opponents.
Amusingly, the Egyptian is apparently only this to the Romans, who he is fighting alongside. There's no indication that the gladiators/rebels have even heard of him.
Star Trek: The Borg Collective in various series. Despite appearing rather infrequently in the series where they first appeared, their sheer implacability, utterly relentless nature, and single-minded goal to assimilate everyone and everything that they come across led to The Federation considering them, as Captain Picard puts in Star Trek: First Contact, "our most lethal enemy". Even Guinan is afraid of them, what with them having assimilated her home world. The suspense that builds up around their incursions into Federation space is particularly indicative of this. It's less so in Star Trek: Voyager, where Voyager encounters them so frequently without being in much danger that the Borg end up exhibiting Villain Decay.
In the first season of TNG, the Ferengi were treated this way. It didn't last long.
The Iconians were so feared that the other races in the galaxy banded together to wipe them out via orbital bombardment of their homeworld and called them "Demons of Air and Darkness". In Star Trek Online, the Iconians turn out to be not so extinct and they are responsible for the strife plaguing the sector. In their first real appearance, a single Iconian vessel wipes out two Borg cubes and leaves just as quickly, telling the survivors to fear them. Furthermore, the Dominion is so afraid of the Iconians that their official policy for dealing with them is to give the Iconians anything and everything they want, hoping they will go away.
Star Trek: Deep Space Nine: Dax is notorious among Trill initiates for being one of the worst mentors for an initiate to be assigned to. Dax is infamous for have broken more initiates (thus ending their chance of becoming Joined Trill) than any other Joined Trill. Being assigned to Dax is therefore regarded as practically a death sentence by most Trill initiates.
A variation occurred in the Star Trek: The Next Generation episode "Face of the Enemy", where Troi was kidnapped by a Romulan underground movement, and subjected to surgery so she could assume the identity of a Romulan operative in the Romulan Tal Shiar named Major Rakal. (The real Rakal had been covertly killed by members of the movement for this very purpose.) Troi quickly found that other members of the Romulan crew were terrified of her because of the reputation of both Rakal and the Tal Shiar; apparently, they have the authority to execute other Romulan officers for any reason they see fit, and are quite liberal (and at times cruel) with this privilege. (In fact, that's the exact reason resistance member N'Vek chose her for the part; he thought that she could play the role well and not give herself away, and in fact, they never suspected a thing until it was too late, with N'Vek making a Heroic Sacrifice to finish the mission and get Troi back to the Enterprise.)
Supernatural: Has a few depending on the season. In the first season demons are nigh unstoppable and in the first episode with one in Sam and Dean have serious trouble exorcising a single demon because it could be anyone, conventional weapons are all useless (the only thing to hurt it is holy water) and to kill it they have to enable it to posses the plane itself. When they learn that going up against the Yellow eyed demon that killed their mother means going up against a lot of demons they become very unnerved. Once they get their hands on the Colt (a gun made to kill demons) and learn how to make Devils traps Demons become a lot loss threatening, though are still highly feared until...
Season 4 shows that Angels are this to Demons: a group of Demons harass Sam and Dean shortly after Dean is mysteriously brought back from the grave but Sam points out that the Demons don't know how Dean is alive either and are more afraid than they are because it means there might be a bigger fish. Later Sam goes back to kill them and finds that their eyes were burnt out because the saw what it really looked like. The only survivor is gibbering about how the end is upon us and how everything is going to die. When it turns out that nothing Sam and Dean have or can do will hurt an Angel and sometimes an Angel can go bad and/or insane things escalate quickly.
Series 7 ups the anti with the Leviathans who can regenerate from just about anything, shape shift into someone perfectly (as long as they have a tiny DNA sample), tear through just about anything and anyone, are immune to Angels, Demons, Monsters, Weapons, Fire, Iron, Salt and Silver. The only things that can hurt them are Borax based cleaning products. Decapitation slows them down and they can only be killed by being eaten by another leviathan, being forced to eat themselves (by putting a bib round their neck which is used a a punishment for failing their leader) or being stabbed by the "bone of righteous mortal, soaked in the blood of the fallen" (a fallen angel, an alpha monster and a demon). The head Leviathan is this to his subordinates as his very name is enough to make them shake in their boots and plans to kill every monster demon and angel he can and change human physiology (making us live longer, more disease resistant and much fatter) and psychology (making us passive) so the whole planet becomes a buffet for the leviathans.
Death tops them all. He is the only thing in creation that can make Dean Freakin' Winchester quake in his boots.
The Wire: Omar Little. An absolutely Badass stick-up man who robs drug dealers for a living. The drug dealers generally see him as more of a force of nature. Even CHRIS AND SNOOP get shaken when they realize he's after them.
By extension, Chris and Snoop also count.
Brother Mouzone is possibly an even bigger one than all other mentioned. Even the thought of bringing this guy into help them scares the shit out of Stringer and Prop Joe.
Though to be fair, only certain people like Joe and Stringer know who Brother Mouzone is. Omar apparently didn't know him. Cheese suffered a great deal a pain and humiliation for his ignorance of the man.