For example, both Barack Obama and John McCain were considered unlikely to even be allowed to run as their party's candidates in the 2008 US presidential election.
On that front, Obama is far more of a darkhorse than McCain. McCain has run for president before, lots of people knew who Mitt Romney was, and everyone knew who Hillary was. Most people outside of Illinois had never even heard of Obama until the middle of 2007. Plus, as many people, even Obama himself, have noted, if you had told people in 2005 that the next president would have the middle name "Hussein" (and a surname that's one letter away from "Osama"), they would have assumed you had a crappy sense of humor.
Similarly, Sarah Palin was virtually unknown outside of Alaska prior to the 2008 election.
And as we approached Super Tuesday, who would have through the Republican front-runners would include Newt Gingrich and Rick Santorum?
Though he was in no position to win the race, congressman Ron Paul is easily the most popular candidate in the 2012 elections to young people, and even to some in the left. Don't believe me? Just compare the numbers in a Romney or Santorum rally to the number of participants in a Ron Paul rally.
Sometimes, it's members of the candidates' families who can become the darkhorses. Laura Bush and Michelle Obama are both pretty well-respected, even by people who don't agree with their husband's policies. Bristol Palin, Megan McCain, and several other children of candidates have also gotten a certain level of popularity for one reason or another.
Boris Johnson, a Cloud Cuckoolander of a politician, managed to become Mayor Of London (and was re-elected as such) purely because of his personality... and his hair. Naturally there are some who dislike him because he can't be taken seriously and shouldn't be in such an important position of power... but that's also the exact reason why some people love him: he's too silly to take seriously.
Dwyane Wade, drafted after Lebron James and Carmelo Anthony. Most people were only focused on James and Anthony and Wade barely got any attention. Wade was the first of them to win a championship and a finals MVP.
The 2001 Seattle Mariners and Arizona Diamondbacks. No big stars and almost everyone was over 30 on the Mariners, who would go on to win the most games in an American League season ever and tie with the 1906 Chicago Cubs for most wins in a season in all of baseball. This coming from a team that had never gotten serious consideration in a post-season before and had never gone to the World Series. The Diamondbacks, meanwhile, were the newest team in baseball and did what neither the Mariners or anyone else in the past three years could do - beat the Yankees and win the World Series,
Just to prove Real Life is just as surreal as fiction, the only person who thought Spain would win was Paul the Psychic Octopus, clearly the Dark Horse of world cup pundits.
The Cheese-Eating Surrender Monkeys of 1998. France failed to make the World Cup in 90 and 94 and got into the 98 tournament only by hosting the dang thing. They shocked the world with one of the most dominating defenses ever (one non-PK goal for the entire tournament) and capped off their improbable win with a 3-0 win over supposedly invincible Brazil in the final.
Greece winning the 2004 European Championship. No one expected them to even win a match, let alone the tournament: they were 80-1 outsiders before it got underway.
Denmark winning the European Championship in 1992. They weren't even supposed to be in the tournament, but replaced Yugoslavia (who was excluded due to UN sanctions) at the last moment. Both players and trainer were thus utterly unprepared and treated the whole thing as an experience.
For other continental soccer/football championships - we have Iraq winning the 2007 Asian Cup (not only during the War, but also overcoming multiple continental giants) and Zambia taking home the 2012 African Nations Cup after a long penalty shootout against the Ivory Coast.
Geoff Hurst was only included in England's 1966 World Cup squad as back-up to the England's established star striker Jimmy Greaves. However Greaves was injured early on, and Hurst went on to score a hat-trick in the final as England won the tournament; he was later knighted. Greaves, on the other hand, never played for England again.
Usain Bolt as recently as 2007 was solely a 200m sprinter who only occasionally ran 100m races for training purposes. By 2009 he was both World and Olympic 100m champion and had reduced the world record from 9.72 seconds to 9.58 seconds, a huge margin for one of the most pursued sports records.
For the NHL, the Phoenix Coyotes have a surprisingly large fanbase, despite its hot climate for the city. It also helps that the Coyotes are The Woobie of the NHL, only winning their first playoff series in Arizonanote As the original Winnipeg Jets, their last playoff victory had been a first-round defeat of Calgary in 1987. in 2012 by beating Chicago, but still loved. Well, at least in the U.S.
During the Sydney 2000 Olympic Games, Australian comedians H.G. Nelson and Roy Slaven, unimpressed with the games' mascots, unveiled their own mascot, Fatso the Fat-Arsed Wombat. Fatso became more popular than any of the official mascots.
Allen Craig earned Ensemble Darkhorse status at the same time for less heroic reasons: the phrase "Win one for Torty," referring to his pet turtle, became a monster Memetic Mutation among the fanbase. Torty Craig, in turn, became a Twitter phenomenon, Tweeting happily about his adventures with his Master Allen.
Limited edition food or beverage flavors can become so unexpectedly popular and profitable that it would be staggeringly insane for a manufacturer to not re-release them. One of the best examples of this in the United States was probably Mountain Dew's Livewire, which added a touch of orange flavor. It was originally supposed to be produced for just a single summer run in 2003, but consumer reactions to it were so overwhelmingly positive that it was brought back a year later, and then eventually produced all year round for several years.
DJ Mbenga during his time in the Los Angeles Lakers was a fan favorite even though he barely had any playtime in the team.
The Olympic Games have athletes whose actual performance was actually rather unremarkable and borderline So Bad, It's Good but was nonetheless remembered more than the actual medalists. Notable examples include Eddie "the Eagle" Edwards (Calgary 1988) and Eric "the Eel" Moussambani (Sydney 2000).
James K. Polk, 11th President of the United States. He started out as a longshot for the Democratic presidential nomination in 1844, behind the three frontrunners, but not only won the nomination, but also the presidency. He is the most effective single-term President in US history in terms of his historical legacy — and the only reason he's single-term is that he decided not to run again in 1848.
Benjamin Franklin could count as this for the United States' Founding Fathers. Though he had a series of prolific careers (as a writer, scientist, inventor, and diplomat) he only had a supporting role in most of the events leading up the the country's founding, and (somewhat famously) never actually held any public office other than Postmaster General of the United States. In spite of this, he's arguably the most beloved and well-known figure in American history, partly because he's just such a fascinating figure.
Similarly, John Hancock and Paul Revere are also exceptionally famous beyond their actual professional accomplishments. Revere is only well-known thanks to a very famous poem, and Hancock was completely forgotten until the country's centennial celebrations, when everyone was trying to research every tiny corner of the Revolutionary period. Some others are also arguable cases (Samuel Adams and Patrick Henry come to mind, but they did have some notable influence in shaping the country), but those two stand out.
A more literal one is Phar Lap, a racehorse during the Great Depression in Australia. Phar Lap was born to two unremarkable horses, and all its siblings were equally unremarkable: only two managed to win any races at all. Not only that, but prior to its racing years, it was covered with warts, gangly, and had an unusual manner of walking. Did we mention it won a Melbourne Cup, two Cox Plates, an AJC Derby and 19 other races?
Brian Scalabrine is a former NBA player of decent skill. However, since he is a ginger, he tends to get a lot of fans and memes.
Sébastien Chabal during his time in the French Rugby team. Absolutely average, he gained much more popularity than the rest of team by way of his fearsome look and his Badass Beard.
Mercedes-Benz follows this trope with the G-Class. In 1979, it was originally a vehicle made for military purposes, but gained steady ground in popularity because of its performance, strength, and easily recognisable box shape. It was supposed to be replaced by the GL-Class, but it got so successful that Mercedes kept it in production and released its successor at the same time. To this day it's Merc's second longest-running and most successful production vehicle for over 35 years.