The term "dark horse" has its origin in horse racing. An unknown horse would be risky to place bets on, compared to a horse with a known track record. Because gamblers would be "in the dark", when an unknown horse won a race, it was called a "Dark Horse Victory." The term is also used in politics to describe a lesser known candidate who does better than expected in an election.
This is used to describe a side character making up part of the ensemble, either a non-lead secondary character or a mere Flat Character, who then becomes unexpectedly popular with the fandom, sometimes even more than the lead characters, depending on who and where the fandom is, as well as what the other characters are like in comparison. For example, the hero is not as popular because they're too much The Everyman. Often, this can happen because the character has very few character traits, which allows fans to imagine what they are. The dark horse can sometimes be viewed as the character equivalent of a Cult Classic.
The writers or producers may be tempted to Retool the work's premise to put them in the spotlight. Sometimes this works, but usually it's a bad idea for two reasons, both relating to what happens when you take a supporting character and move them into The Protagonist's position. The first is that writers often "adjust" the character so that they can fit into a conventionally heroic role. In the process, this can destroy the unconventional traits that made the character a dark horse in the first place. The second is that if the writers don't do this, traits that were entertaining in a secondary character may become grating and unpleasant in the protagonist.
However, it's still good business to bring dark horse characters back, even if they were originally meant to be featured for only a short time. Thus, episodes which do not specifically require a certain character will be more likely to use them.
Occasionally, if an antagonist becomes a dark horse, the writer may decide to have them perform a HeelFace Turn in situations where the only other option is being killed by the protagonists. However, if the series doesn't have an end planned, it's more likely that they'll just escape.
If the dark horse becomes an important character, they're now a Breakout Character. If they're so minor that they only show up for one or two scenes through the entire work, then they're also a One-Scene Wonder. See also Adaptational Badass, Ascended Extra, Memetic Bystander, Lower-Deck Episode, A Day in the Limelight, and Unpopular Popular Character. Creator's Pet is the polar opposite, a character who the writer grows fond of, but the fans do not. An antagonist who becomes popular despite the author's intentions is Draco in Leather Pants, which is an example of Misaimed Fandom. The natural extension of this is the Spotlight-Stealing Squad. Major characters who end up overshadowing their castmates often fall under Face of the Band, whether they're the true lead character or a supporting member who ends up overshadowing the lead, à la Darth Vader or Stewie Griffin.
Although this applies to individual characters, as a YMMV item, it should not be listed on character pages. This seems counterintuitive, but character pages are meant to list tropes audience members can see in the work in question, not fan opinion expressed outside of the work.
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- Motu Patlu: Inspector Chingum has gained the hearts of many with his hammy, over-the-top personality and incompetence. It gets to the point that he got his own self-titled spin-off show in 2018.
- An episode of Simple Samosa features a 15-second-long appearance from a sugar cube musician named Suga, from the band Ghee-TS. Fans of the original K-pop band the character is referencing quickly noticed, despite his small amount of screen time.
- The Metamor City podcast has Artax, originally created as an expository engine to explain some of the more complex bits of the world's magic in universe, the fans have often cited him as a favorite character over several of the main characters.
- The Engineer from True Capitalist Radio. Many of the trolls who call into the show wish that he would host more than Ghost. Some of the trolls have gained popularity after the show's renaming to True Capitalist Radio. The Internet Buttstalker, Tub Guy, Ghetto Capitalist, Horny the Clown and the Ban All Bronies guy/Btown also are popular trolls.
- Due to the nature of the show, a case could be made that anyone besides The Narrator Cecil (and maybe his eventual boyfriend, Carlos the Scientist) from Welcome to Night Vale as this. However, the stand-out by far is Tamika Flynn, the brave twelve-year year old girl who led all of Night Vale's children through the dreaded summer reading program, wears a librarian's severed hand as a necklace, and has formed a militia army of children to fight against Strexcorp.
Cecil: If station management is listening, I, of course, hope we find Tamika Flynn and bring her home safely. I hope she finds you first, that is.
- Random Assault: Mela Firefly. In a poll on the show's Facebook page for which guest hosts fans want to see again, she was the highest-voted.
- In The Scathing Atheist Podcast, the crew had a regular segment reviewing terrible religious films, where they brought on their friend Eli Bosnick as a guest. Eli's improvised insults towards the characters and the writing brought such high demand from the fans that they made an entirely separate podcast, God Awful Movies. They spread a crowdfunding campaign on Patreon to fund the show and met their goal in two days.
- Survival of the Fittest:
- Rosa Fiametta seems to be one of the most liked characters for v4 so far, alongside being a Launcher of a Thousand Ships. As well Orn "Dutchy" Ayers, a free-loving, comic-reading, foreign very sensitive, Icelandic immigrant, who was held in high regard by many of the handlers before he even appeared for real.
- Jared Clayton of SOTF-TV is also pretty popular, considering that he isn't even a contestant, just a mentor and has had all of one line (not even onscreen!) in the actual game. This is probably due to the in-character twitter the staff maintains for him, featuring snark galore.
- Mr. Kwong, a teacher in v4. It's very unusual for a non-student to get much attention (either in or out of character), but you'd be hard pressed to find a handler that doesn't like him.
- From Dino Attack RPG:
- Trouble's popularity was what got him promoted from what should have been a one-shot character to one of Rex's best friends. It is also noteworthy to mention that, during his long-term absence, there was quite a bit of demand for his return.
- Dust, the ultimate Magnificent Bastard with a complex personality and unique motives, quickly became one of the RPG's most popular characters. In fact, he is often hailed as the best-written character in the whole RPG.
- Montoya, who served as a Foil to the rather unlikeable Trigger, proved to be much more popular than his partner by acting much more human, which earned him the audience's sympathy.