With this said, I'm gonna pimp a few runners, as that's my sport of choice and I know a lot of them will be relative unknowns.
Emil Zatopek - Best known for winning 3 gold medals at the 1952 Olympics in 5000m, 10000m & the marathon. The marathon is the most notable as it was the first one he ever ran. One of the first athletes entered into the IAAF Hall of Fame. Well known for his brutal training regime and awkward running style, a legend in running history.
Ann Trason - Western States ultramarathon legend. For those unaware, ultramarathons are races longer than 50 miles, a lot of them clocking 100. Between 1989 & 2003, she won the female race 14 times, also holding the female course record for 18 years. She's also held the female course record for the Leadville 100 since 1994, as well as course records still standing in 10 other ultras.
Scott Jurek - Another Ultra god, winning the Badwater Ultra twice, Spartathlon (a 153 mile race!) three times, and the Western States 7 years straight between 1999 & 2005. Oh and he did all this on a strict Vegan diet.
Insane that a guy who never ran a marathon before wins the gold medal.