Pro wrestling features an elaborate set of character alignments written by the people who create the show. A Heel is the bad guy. The Face is the good guy. A Tweener is a character who shares aspects of both sides in their character, or one who gets cheered like a face despite behaving like a heel.
However, despite what the creators give their wrestlers in terms of characterisation, it is up to the fans of the promotion to actually follow through. The best wrestlers and writers can push the characters in a certain direction to achieve the specific heel/face reception from the supporters but the fans may not take to the wrestler like they expect.
This can take the form of X-Pac Heat, where instead of disliking the character because of their heel behaviour, they simply dislike the performer themselves and don't want them showing up at all.
Being an Ensemble Darkhorse in pro wrestling is the opposite of that. It can take the form of a character or wrestler being viewed as underused or undervalued by the fans of the promotion. Or a character who is popular regardless of what alignment they play because they can do both roles well.
Note, that because of the nature of the industry, becoming a darkhorse can easily lead to a 'push' from the company as they seek to take advantage of that popularity by increasing the wrestlers visibility to gain higher ratings, pay per view buys and merchandise sales.
Masato Tanaka got over quicker with ECW's mutants than anyone before him ever had, to the admitted surprise of Shane Douglas even, who usually has nothing nice to say about anyone.
AJ Lee started as a losing WWE NXT entrant (which was fan voted), but after Daniel Bryan shot her into the main event became wildly popular. Once the creative team recognized that, she was given a General Manager role on Raw, followed by winning the WWE Divas' Title soon after.
Her former Chick Busters partner Kaitlyn as well. Despite being the least trained and least capable wrestler, it showing, and admitting it, she bagged the most votes to win NXT Season 3 and proved popular enough with fans to push her for the Divas' Championship against Eve.
Christian always had a following far greater than his mid-table position in WWE suggested.
Dolph Ziggler was supposedly a heel all the way from his debut until he won the World Heavyweight Title the night after WrestleMania. Not that you would've noticed from the booming cheers when it happened.
Diabolic Khaos always had more supports than a duo (then quartet) of demonic vagabonds with few redeeming qualities and fewer social skills would suggest, especially at Gateway Championship's "Adult" shows and especiallyMsChif, who was repeatedly voted by fans to be GCW's best wrestler. Delirious, despite being nothing more than a jobber in TNA and Ring of Honor, remained getting massively positive responses whenever he showed up, leading to him getting his own angles in ROH (Jim Cornette threatening to fire him if he didn't stop losing, Hang Men 3 turning him crazy by trying to kill him, feuds with Rhett Titus also involving Daizee Haze and Jerry Lynn, a prominent spot in Age Of The Fall, ect)
While Alexis Laree was popular every she didn't have a full on heel gimmick, not even that stopped WWE fans from loving psycho lesbianMickie James.
Stevie Richards in WWE. On the September 10, 2007 Raw,Vince McMahon's flunky Jonathan Coachman told him that, in the online poll on WWE's website of who the fans thought would be revealed to be Vince's "son," Richards was in second place, leading to Coach bringing Richards in to see Vince. Considering how Richards had not held a title for five yearsnote He scored his 22nd, and final, win of the WWE Hardcore Title for less than a day on August 18, 2002 and had not even had a PPV matchnote The Mexicools (Juventud Guerrera, Psicosis [who qualifies for this himself] and Super Crazy) d. the Blue World Order (Richards, The Blue Meanie and Nova) at WWE The Great American Bash 2005 on July 24, 2005 in two years at that point, it just showed how Richards had been able to retain a fan base from his years in ECW.
Part of the reason for Richards' popularity was he was on Sunday Night Heat for years without being fired, and had an ironic gimmick about his lack of popularity.
Paul Burchill's pirate gimmick was immensely popular, yet he was not used well at all. When Vince McMahon, who didn't get the gimmick, made Burchill drop it, there was a widespread outcry from fans. This didn't stop McMahon having his way.
Al Snow started his career in 1982, but didn't really get much television exposure until he arrived in SMW in 1994-1995. He spent years jobbing in WWE under failed gimmicks (Avatar and Leif Cassidy of the New Rockers) until he got shipped down to ECW, where he started the "Head" gimmick, which got him over. Even with this newfound popularity and recognition, he still has only one PPV main event to his name (a loss to ECW World Heavyweight Champion"The Franchise" Shane Douglas at ECW WrestlePalooza 98, May 3, 1998.)
After Melina went out on her own from MNM and eventually turned face, she received great reactions from fans. She began to rival Mickie James in terms of the pops she would get - and this was after she had spent several months on the shelf due to injury just after her face turn.
Backstage interviewer Renee Young is actually incredibly popular with fans. It helps that she is both very good at her job and is a genuine wrestling fan.
Despite what Vince Russo may want you to believe, nearly every luchador in WCW had a pretty easy time getting over, no matter their usage. Examples would be Silver King and El Dandy, icons in Mexico who never received notable pushes or much to do but have still stayed remembered by American fans. But nobody got more bang for their buck than La Parka, who has been a star in every promotion he's ever worked, including WCW. Even Kevin Nash, who couldn't book his way out of a paper bag, decided that he was going to put him in the main event of his last show as head booker in 1999, because Parka was just that over.
The entire WCW Cruiserweight Division, despite often being pushed aside and marginalized on the shows themselves, remained popular with the crowds. Dean Malenko, Chris Jericho and Billy Kidman in particular. Even Chris Benoit, who really was accurate to what Kevin Nash dubbed vanilla midgets, would prove popular enough to ride to the main event when given the shot in WWE. Eddie Guerrero retained his popularity as well, even after seven years as a jobber.
Shark Boy has been this almost everywhere he has been, even in TNA where he was rarely used and mainly as a jobber when he was. His "Stone Cold" was about the only role he took that wasn't universally received but the Impact Zone still loved it.
What was seemingly a roll with Replacement Scrappy written all over it, substituting for Aja Kong, Amazing Kong quickly took off to the point game shows were requesting her and some fans were calling for a World Heavyweight Championship run after two appearances in TNA, which consisted of squashingGail Kim and being eliminated in a battle royal.
Daffney became surprisingly popular in TNA despite degenerating into a rarely used jobberheel as time went on, devoid of any redeeming qualities. Following her governor run she had a (weak) Freudian excuse and that was it.
Kaientai's hilarious over the top evil gimmick made them incredibly over, coupled with the fact that they were 2 of the best wrestlers in the WWE at the time (Mick Foley is on record stating that if the WWE title was actually based on talent, Taka would be the champion). Unfortunately, being over made them unusable as jobbers, and resulted in the team being broken up.
Referee Paul Turner is amusingly popular with Ring Of Honor fans, considering he got that way from stopping a fan from hitting top heel Austin Aries.
Similarly, the Eagles Club loved Senior Shimmer referee Bryce Remsburg after he protected Allison Danger from Portia Perez while Danger was pregnant.
A brief overview of UltraMantis Black's career, which involved constantly losing in title shots for ten years straight, proclaiming his disgust of the human race and love for communism, starting a religion of evil and brainwashing the aforementioned Delirious wouldn't give one an idea of just how popular he is with Chikara's fans.
Necro Butcher caught on better in Ring Of Honor than IWA Mid-South, which given the latter being in a Garbage Wrestling phase you'd expect to be the opposite. Even after he joined the initially X-Pac Heat drawing Age Of The Fall Necro Butcher did not suffer much loss of popularity.
Ethan Carter III's popularity bears mentioning considering he was in nothing but irrelevant squash matches in what felt like all year. Even follow two pay per view "feuds" he still had only had two matches that anyone could consider worth watching. What worked for him were his promos and video packages being hilarious. His partner, Rock Star Spud also qualifies, despite having even less in ring credibility to his name, mainly existing to serve as a meat shield for Ethan Carter and whoever else TNA wants to milk an impending encounter with. Again, what works is people finding him funny.
Michael Elgin early on was considered a generic power wrestler distinguishable only by his mullet, but he rode the wave of positive responses following a surprisingly good match with Davey Richards to the ROH World Championship at Best In The World.
Despite their status as jobbers, 3MB (Heath Slater, Jinder Mahal, and Drew McIntyre) has a pretty big following and is the subject of the "Reasons to Like 3MB" meme.
Outside of 3MB, Heath Slater is an Ensemble Darkhorse in his own right.
Pinup lady Rayna Von Tosh, whose wrestling appearances were sporadic at best, tends to be spoken of fondly despite hardly ever being seen. Particularly by fans of Shimmer, where she has only had five matches but still got return request after a 31 volume absence. That's four years for those who don't follow.
La Rosa Negra's responses in The Orpheum can be explained by it being in her stomping grounds; those of everyone outside cannot. She had no wins, promo time, video packages, and little individual focus in 3-4 ways with those who did, yet had comment sections, forums and even professional reviewers abuzz with questions. Her first real Shine angle with Shanna got more attention than her main event work elsewhere.
A developmental example. When footage from an NXT promo class leaked online, several promos saw developmental Diva Alexa Bliss playing the girlfriend of redneck Scott Dawson. She got lots of praise from fans, who eagerly awaited her debut. Although she had a different gimmick when she did debut, fans still remembered her and immediately chanted "let's go Bliss!"