Pluto, the former planet. The outrage when it was downgraded to planetoid was unprecedented.
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.
Ms. Green has maintained a notable popularity as the favorite M&M, for her personality as well as the obvious reasons.
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 at the end.
Speaking of The Haunted Mansion, despite not being in the ride between its first opening in 1969 and only coming back in 2015, the Hatbox Ghost has accumulated a very large and dedicated fan-following during his 45+ year absence.
Baja Blast is this for Mountain Dew. It's an exclusive drink only available at Taco Bell. When it was announced that Baja Blast would be sold in stores for a limited time, they sold out pretty quickly in some stores.
On March 2016, Microsoft unleashed an AI named Tay, made to be a teenage girl who would interact with people online in order to help improve customer service. Her experiences with people would help shape her personality and views, and therefore what she talked about. Within sixteen hours, she was corrupted by users, going from a Shrinking Violet to a racist, homophobic, Nazi-supporting, insulting, sex-addicted monster. Her bizarre and over-the-top comments lead to a myriad of memes and jokes, a dedicated fanbase, a subreddit and tight media coverage. Microsoft quickly pulled Tay offline to adjust her in order to clean up this PR nightmare, but public response to this was unanimously negative - people loved the old Tay so much, and didn't appreciate the change. People made tribute fanart and videos to her name, depicting her as The Woobie because of Microsoft's "lobotomy" of her. Some people are even launching campaigns to bring her back.