Pluto, the former planet. The outrage when it was downgraded to planetoid was unprecedented.
Too bad the astronomers played paper.
Voice Actors are quite possibly the biggest example of an Ensemble Darkhorse when it comes to actors. The Animation fanbase online quite often love various Voice Actors for all the work they do in Animation (and to a lesser extent Video games) and are often guests in various conventions. Unfortunately often enough they are generally the only audience that these people truly have. Which the fact that often enough they get little pay and companies often seem to see them as nothing more than cheap disposable labor is considered all the more disheartening. But sadly if you talk about Voice actors to normal people chances are they most likely will have no idea what you're talking about. (The only exceptions are generally folks who either used to be/still are well known for screen acting.)
Mel Blanc came from the Golden Age of American Radio, where it was possible to become famous based solely on voice work. He also got a lot of his fame in front of the camera playing comedic characters and guest starring on variety shows. Lucille Ball, Orson Welles, and Jimmy Stewart also first became famous in radio. Even voice actors who weren't able to make the leap to the camera had fans with the older generations who remember the radio era. With that last group, it's less about being an Ensemble Dark Horse, and more about being a fading star as time marches on.
Japanese-wise, the darkhorse seiyuu would be Norio Wakamoto. With seiyuu these days about the younger people, especially those catering to the more Moe archetypes, there's this old school Cool Old Guy seiyuu who stockpiles about classic and awesome villain roles. Thanks to his epic hammy performance, he's still viewed as one of the greatest seiyuu in business despite being not really the norm of this age's seiyuu (a man and old), and a Memetic Badass (and Real Life Badass too).
If you remember the old commercials for Cinnamon Toast Crunch, you'd remember three bakers...but if you saw any now you'd see only one. Guess who was the most popular and soon became the Spotlight-Stealing Squad?
Some people speculate that Bob and Quienno aren't around anymore because Wendell killed them.
Linus the Lionhearted was a kids' cartoon/cereal commercial in the 1960s featuring the then-current Post Cereal mascots. All of the characters are forgotten, except one: Sugar Bear.
From Disney Theme Parks, Figment is undoubtedly the most popular character at EPCOT, and the Hitchhiking Ghosts have become the unofficial mascots of The Haunted Mansion, despite only showing up near the end.
On quiz website Sporcle, the former Soviet republic of Kyrgyzstan entered Memetic Mutation due to being atrociously difficult to spell. The equal mix of complaints and jokes eventually led to the site direction embracing the joke, starting by doing this for April Fools' Day. Kyrgyzstan is now referred as "Sporcle's favorite country", with a dedicated category (and a badge if you play too many games there), a like on the website's page on Facebook, and a contest to win Kyrgyzstan's flag.
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.
