BJ Penn comes to mind.
but I am thinking about his BJJ... he was the first American to win gold at a World Jiu-Jitsu Championship, and if I am not mistaken, in 3 years or so (which is what rendered his nickname as prodigy)
Edit: I stand corrected. From Wiki: "The Prodigy originates prior to him competing in mixed martial arts, from accomplishing his extraordinary feat in the sport of Brazilian Jiu Jitsu (earning his black belt in just under three years and winning the black belt division in the World Jiu-Jitsu Championship only three weeks later)"