These guys fancied a career change and found themselves in one of the toughest sports in the world.
CM Punk
The most high-profile crossover to professional fighting in recent years, former WWE star CM Punk's tumultuous UFC stint will likely serve as a warning to any celebrity that might be tempted to test their skills inside the cage.
Punk's transition to the world of MMA could not have come at a bigger time during his career. After rising from obscurity to become one of the biggest names in professional wrestling, Punk had a very public falling out with the higher-ups at WWE before deciding to seek pastures new.
Though Punk's MMA fandom was well-known, the news that he had signed with the UFC in 2014 came as a total surprise to the entire sports world.
Ultimately though, Punk's transition to the Octagon would not prove to be a successful one. In his first bout, he found himself completely outmatched against Mickey Gall and was summarily submitted in the first round. Punk's sophomore effort came against a more closely matched opponent in journalist Mike Jackson, but the former WWE Champion once again tasted defeat, this time by unanimous judges decision.
Jose Canseco
One of the most controversial figures in sporting history, Jose Canseco's legendary career in Major League Baseball was marred in controversy after his retirement when he became one of the central figures in the sport's performance-enhancing drugs scandal.
With a background in taekwondo and karate, Canseco decided to take his career in a different direction after retiring from baseball and transitioned to MMA by entering the DREAM Openweight Grand Prix and facing off against South Korean kickboxer Hong Man-Choi in 2009.
In what was an egregious mismatch, Canseco was easily defeated by the 7ft tall fighter as he submitted to punches after just 77 seconds.
Professional combat sports weren't a one-and-done for Canseco either aged 56, the former Oakland Athletics star took part in a boxing match against Barstool Sports' Billy Football earlier this year. However, the Cuban-American failed to better his first attempt at professional fighting and was defeated by way of first-round knockout.
Greg Hardy
After rising to prominence for his exploits as a defensive end for the Carolina Panthers and Dallas Cowboys in the NFL, Greg Hardy's gridiron career would come to an unceremonious end after he was found guilty of domestic violence in 2014, and possession of cocaine two years later.
After his career failed to recover from the controversies, Hardy swapped the football field for the cage and began training in MMA, making his debut just one year later.
Despite having no background in combat sports, Hardy immediately set himself apart from some of his contemporaries on this list by utilising his insane athleticism to his advantage as a fighter. After compiling a 3-0 record as an amateur, Hardy was signed to a developmental deal with the UFC.
Since then, the University of Mississippi alumn has compiled a respectable 4-3 (1) record in the UFC, often utilising his physical gifts to make up for his technical limitations in the Octagon.
James Toney
A bonafide world champion in three different weight classes and one of the most defensively skilled fighters in the history of boxing, James Toney established himself as one of the premier talents of the sport during the 1990s and early 2000s.
Since then, UFC stars such as Holly Holm have proven that, with the right training, a talented boxer can eventually develop the requisite skills to successfully transition from the squared circle to the Octagon.
However, when Toney signed with the UFC in 2010 to face off against MMA legend Randy Couture, his transition did not come as the cumulation of years of training to round out his grappling skills, and instead was seen by many as little more than a cash grab.
Predictably, the then-42-year-old Toney was unable to bridge the vast disparity in grappling skills between himself and the former UFC Heavyweight Champion Couture with less than a year's worth of training and was submitted in the first round.
Alexander Volkanovski
So successful has Alexander Volkanovski's transition to professional fighting been that many MMA fans don't even realise that the current UFC Featherweight Champion was a professional sportsman before calling the Octagon home.
Despite now being known as one of the most fearsome competitors in the lighter weight classes of the UFC, Volkanovski tipped the scales at a whopping 240lbs whilst playing semi-professional rugby in his native Australia back in the early 2010s.
After starting training the various facets of mixed martial arts at 22, Volkanovski would continue his remarkable transformation, making his amateur fighting debut at middleweight before finding his way down to his current fighting weight of 145lbs.
Unlike some of his contemporaries on this list, Volkanovski's transition to the world of MMA has proven to be a hugely successful one. Just seven years after making his professional debut, the Aussie would reach the pinnacle of the sport, defeating the legendary Max Holloway to capture UFC gold at UFC 245.
