Rocky Marciano held the unbeaten record of 49-0 for over 50 years. A handful of boxers passed that mark only to lose eventually. Currently two who have passed that record are still active.
IBF and WBC Lightweight Champion Mike Garcia is 39-0 and scheduled on March 16th in 2019 to fight IBF Welterweight Champion Errol Spence, Jr., 24-0 for his titles.
When former world champion Floyd Mayweather, Jr. is allowed to fight amateurs like Conor McGregor and have the Nevada Commission sanction it this allowed Mayweather to surpass Marciano at 50-0, and put a lot of money into the Nevada Commission.
Mayweather has talked about fighting another one or two MMA boxers but was told by even Nevada they won’t be able to sanction it again.
Few people seem to know that WBC Minimumweight Champion Chayaphon Moonsri aka Wanheng Menayothin from Thailand is 52-0 as of November 2018 and that last win was over a 2-9-1 opponent in a 6 round non-title bout. I gather he figured if Mayweather can get away with it why hot him? At least his 50 and 51st wins were in world title defenses. All his 52 bouts have been in Thailand since turning professional in January of 2007. He’s appeared in 11 world title bouts.
Former WBO, WBC, IBF and WBA Super Middleweight Champion Joe Calzaghe retired after posting a 46-0 record due to problems with his hands being broken so many times.
Others who surpassed 49-0 before tasting defeat were Willie Pep who prior to Marciano was at 60-0 before losing to Sammy Angott in March of 1943. Julio Cesar Chavez was 87-0 when he was held to a draw by Pernell Whitaker in September of 1993 and three fights later tasting defeat.
Luis “Yuri Boy” Campas was 62-0 when he lost to Felix “Tito” Trinidad in September of 1994. Nina LaRocca was 56-0 when he lost to Gilles Elbilia in February of 1984. Brian Nielsen was 49-0 when he lost to Dicky Ryan in December of 2000.
Moonsri at 52-0 has not tried to unify with the other three champions as far as this writer knows. With his last bout a non-title one in November he will possibly be due to defend his title in February since his last defense was in August.