I'm not talking about superior athletes like Babe Ruth or Wayne Gretzky who changed their sports by their very existence. These are people whose innovations changed the way the sport was done. (I was going to write this earlier but Donald Trump couldn't keep his pie hole shut.)
Candy Cummings - Curveball (Baseball)
In 1867, a small, skinny teenager was playing for an amateur baseball club in Brooklyn and experimenting with different ways to throw seashells when he discovered a way to make the shells curve. Being a pitcher, he tried to do the same thing with a baseball and it did. Problem was he couldn't use it in a game because his catcher, as was standard practice, stood twenty feet behind the batter and a curving pitch would skip past him.
In 1870, a new catcher, Nat Hicks, brought the new technique of crouching right behind the batter where he could catch Candy's curves. Batters have been cursing the curveball ever since.
Duke Kahanamoku - Modern Freestyle (Swimming)
In 1844, two Native Americans traveled to England where they competed in a London swimming race. Flying Gull won easily using a crawl stroke that outraged the European swimmers. The European swimmers used the genteel breaststroke and rejected the heathen stroke that splashed "grotesquely." A stroke similar to what Katie Ledecky uses.
Later swimmers like Brit John Trudgen and Aussie Dick Cavill relearned the stroke from Native Americans and Pacific Islanders. American swimmer Charlie Daniels added the "six-beat kick" while winning early Olympic contests. In the 1912 Olympics Duke further modified the stroke into the freestyle form still used today. Oh, and Duke introduced surfing to the world, so there's that too.
Sonja Henie and Dick Button - Figure Skating
Prior to Sonja Henie women's figure skating was mostly carving highly specific designs. Ladies wore long thick dresses and moved around the ice purposely if not gracefully. Sonja changed that during the 1920's. She wore short skirts, part ballerina and part flapper. And she moved with style, flair, artistry, and elegance.
Ulrich Salchow invented the jump that bears his name in 1909, towards the end of his career, but it took Dick Button in post WWII competitions to demonstrate that skaters can add power to grace. He landed the first triple jump and invented new routines like the flying camel.
Jacques Plante - Goalie Mask (Hockey)
In 1959, the Montreal Canadians goalie was hit in the face by a shot, breaking his nose. After a quick stitch-up Plante returned to the ice wearing a hand carved mask he had been wearing during practices. The team's coach hated the mask but he had to allow Plante to wear it because at the time NHL teams seldom dressed a backup goaltender. The mask was slow to be accepted by goalies because of tradition and whatever is the Canadian word for machismo. Now that the mask has evolved to something close to a welder's helmet, it has fundamentally changed the sport.
Prior to the mask goaltenders stayed on their feet because dropping to your knees to block a slapshot was an invitation to having your face redesigned by frozen vulcanized rubber. With the mask goalies can fearlessly be on their knees with their legs spread in the "butterfly" technique making it virtually impossible to score a goal along the ice.
Dick Fosbury - Fosbury Flop (High Jump)
It's been almost forty years since any champion high jumper has gone over the bar face down. In 1965, a 16 year-old high school hippie started experimenting with new techniques because he couldn't master the traditional side-straddle jumping style. He found he jumped better going over headfirst and upside down. For years his high school and college coaches tried to get him back to tradition but they couldn't argue with his success at setting school and meet records.
The unknown Fosbury went to the 1968 Mexico City Olympics and set a new world record. Within a couple of years most high jumpers had switched to the easier and more efficient flop.