Named by Democracy

"I was tall, it's true, all of six foot two. They broke me across my back! My name unknown, it's not my own! They call me Crooked Jack."

—Irish Folk song

Perhaps the original is too hard to pronounce, maybe the subject in question simply doesn't have one at all, refuses to tell, or just forgot it. For whatever reason the masses decided on a fitting name for the subject and are sticking to it regardless of any protests. They've been named by democracy. Compare Verbal Tic Name, where the subject will really have a hard time protesting this, and Appropriated Appellation, where someone will adopt their title from someone else's guess or mistake. The naming of the subject has to be depicted or described to be this trope, if we don't know how it came to be it is more likely Known Only By Their Nickname or Everyone Calls Him "Barkeep".

Examples of Named by Democracy include:

Anime and Manga

Scar from Fullmetal Alchemist, his real Ishbalan name was never told and Scar since it was one of the few particular characteristics they could obtain from witnesses at first. He seems to have adopted it to a some degree since he has done many things that go against the Ishbal Teachings (mainly murder).

Scar: Long ago this body did have a holy name given by god, but that person died years ago from a life of sin, malice and retribution.
[not to confuse it with That Man Is Dead]

In Azumanga Daioh, Ayumu Kasuga is quickly nicknamed Osaka (after the city she just transferred from) in spite of her protests. By Year Two, this name catches on to the extent that even the teacher forgets her real name. At one point she even forgets what her real name is.
This is basically how 'wolves' are given their nom de geurres in Ben-To; usually as a result of embarrassing situations that the 'wolf' in question is caught in. For instance, Satou ended up becoming known as 'The Freak/Pervert' after being seen in public wearing women's clothing (against his will, yes, but not that that would help him). It later turns out that Shaga got her name 'Beauty by the Lake' after she fell asleep in the park after netting her first half-price meal, and Yarizui became known as the 'Ice Witch' when she unwittingly attempted to purchase a vodka mixer to go with her meal, under the impression at the time that it was a soft drink.
In Soul Eater Not!, it's revealed that students at DWMA can have their nickname treated as their official name—even if they don't want to, as happened with "Eternal Feather". And they won't even be allowed to change it for a full semester...

Comic Books

DC Comics' Solomon Grundy was named after the nursery rhyme when he told someone he didn't have a name, and had emerged fully grown from the swamp on a Monday.
The Incredible Hulk was an example of this trope but it's since been retconned away.
The people start calling the thing that claimed to be from beyond as the "Beyonder" in Secret Wars.
Superman villain Doomsday technically doesn't even have a name (the closest thing would be "The Ultimate Lifeform" given to him by his creator). The name came from Booster Gold describing the creature's rampage in his first appearance as being "like the arrival of Doomsday", which then spread through the media and became the creature's name amongst the Justice League.
Wonder Woman foe Genocide does not appreciate the name democracy chose for her. She offered no alternative though so people kept calling her Genocide.
Numerous people kept calling Anya Corazon Spider-girl no matter how many times she corrected them. The codename she chose for herself in the web corps was "Araña" and even after she got kicked out it is still the title she prefers but Anya eventually relented and stopped correcting people when she realized it wasn't working.

Film

Who Am I: The protagonist is asked who he is in a language he can't understand. As he is amnesiac he demands to know, "Who am I?". The locals just think he's hammy and shout "Who Am I" with equal enthusiasm when they want his attention.
In Tremors, the townspeople have a spirited discussion about what to call the underground monsters menacing them all, finally deciding on "graboids". The person who came up with the name is killed soon after.
Saw II. John Kramer introduces himself to Eric Matthews using his real name. When Matthews says he thought John preferred to go by "Jigsaw", he says that's just the name the media stuck him with.

Literature

The Black Tower of The Wheel of Time is an example of a place whose name was agreed upon by its residents, to differentiate from the white tower their counterparts work at.

Live Action TV

Babylon 5: Some aliens gloat that their name is a thousand words long and cannot be pronounced by puny Humans. Then they whinge because puny Humans keep calling them "Shadows".
In Spartacus: Blood and Sand, after seeing the title character's performance in the arena, the Romans name him "Spartacus" after a legendary Thracian king. He is at first insulted, and tries to introduce himself, but is interrupted each time. He eventually accepts the name.

Professional Wrestling

After Evan Bourne and Kofi Kingston became tag team champions a poll was held on Twitter to decide on what the new tag team would be called. Air Boom has been derided by some but democracy had spoken.
On their website the WWE asked their fans to vote for names of the various shows their network would air.

Video Games

One Tekken character told the orphans his tag team partner looked after that he didn't have a name they could hope to pronounce. Their response? "You do now!" It was Armor King.
Chaos Lord from Digimon World is named by the other Digimon, which is notable because Digimon usually don't give individual names to anyone.
Garrus of Mass Effect, when anonymously doing good works around Omega, is called Archangel. When Shepard finds him, he asks you to just call him Garrus.

Web Comics

El Goonish Shive had the local mysterious Flying Brick nicknamed "Cheerleadra" by Internet soon after the first sighting, thanks to her outfit. Elliot was busy escaping under cover of more shapeshifting, and blissfully unaware of this development. Until in the home of his pal who knows more about transformations, and whose father as a local wizard-MIB member is always ready to chew out the magic-using teens who fail at forethought. Even then, Elliot was not happy to learn about the name.

Western Animation

Samurai Jack: The time displaced hero was called "Jack" by the first friendly people he met in the future, who were using the term "jack" in the same vein as "dude" or "bro." Jack, however, ends up adopting it as his name, never revealing his given name at any point in the show.
The titular character of Danny Phantom gets rather annoyed that after The Masquerade is blown everyone around town knows him as "Inviso-Bill" until he sets the record straight in the first Made for TV Movie.

Real Life

Dave Sim (creator of Cerebus the Aardvark) once mentioned how several grown-up members of his family spent several hours naming the new cat, finally settling on "Puss". He took this as a sign that Democracy Is Bad.
Emperor Gaius Julius Caesar Augustus Germanicus, known by the nickname given to him as a child dancing for the armies in Germany in a little soldier's outfit, "Little Boots", ie, Caligula. According to Seneca the Younger, Gaius hated the nickname "Caligula", and punished anyone who used it during his reign...yet he couldn't stop the masses.