Fighters who never lived up to their nickname

[Ron Swanson]

Ricky Hitman Hatton, as far as I know he hasn’t killed anyone, yet

[powerpuncher]

Keith “one time” Thurman unless the one time means he only fights once a year. Then it fits

[El Kabong]

Keith “one time” Thurman unless the one time means he only fights once a year. Then it fits

Maybe best used in the context: There was this One Time where Keith Thurman got beaten up by an old Manny Pacquiao.