Regret’s owner, Harry Payne Whitney, liked to challenge visitors to pick her out of a group of male horses; he thought her success was due in part to the fact that she didn’t look feminine. She was 16 hands high and bright chestnut, with a white blaze, a “filly of remarkable length,” as one Daily Racing Form reporter wrote.

It has been a hundred years since Regret was the favorite to win the 1915 Kentucky Derby. She had swept the the Sanford, the Hopeful, and the Saratoga Special the year before. That Derby was the largest ever run, with 16 horses entered, and Regret was the 15th filly entered in a Derby since the first, in 1875. None had ever won.

There was a four-minute delay as the starters worked to get all the horses facing in the same direction, and then they were off. Regret led the whole way and won easily, a length ahead of her nearest rival. “It was so easy, in fact, that I can’t count it among my greatest racing thrills,” said jockey Joe Notter, years later. Some locals brooded; New Yorkers, not Kentuckians, owned the first three finishers. “Has it come to pass that the eastern invaders are to appropriate all the spoils?” asked M. M. Leach.

“I do not care if she never wins another race,” Whitney said, smiling, “nor if she never starts in another race, she has won the greatest race in America and I am satisfied.”

But if the race itself wasn’t thrilling, the idea of a filly as a Kentucky Derby winner more than made up for it. There had been a saying in Kentucky that a filly couldn’t win the Derby; Regret proved that wrong. “Regret Gives ‘Old Man Tradition’ A Good Poke In The Nose When She Wins Kentucky Derby From Great Field of Sixteen,” ran a Louisville Herald headline. The Daily Racing Form’s story about the race said that the 1915 Derby was “the greatest day in the long and honorable history of beautiful Churchill Downs, for it marked the achievement of what had come to be regarded as nigh impossible, a filly won the Kentucky Derby. Yes, a filly.” The win caught people’s imagination; a tobacconist immediately fashioned Regret-branded cigars. Matt Winn, Churchill Downs’ manager, thought that Regret’s win rocketed the Derby to national prominence. “It needed only a victory by Regret to create for us some coast-to-coast publicity, and Regret did not fail us,” he wrote in his autobiography.

Regret’s Derby collided with national events, too. The day before the race, a U-boat sank the Lusitania, with Harry Payne Whitney’s brother-in-law Alfred Vanderbilt aboard. A horseman himself, Vanderbilt had sailed for England to offer the British army a fleet of wagons and his services as a driver. “He said he felt every day that he was not doing enough,” a surviving passenger said.

At first, there was some confusion over whether Vanderbilt had died; races were run on Derby day, May 8, at the Whitneys’ Long Island farm, Meadow Brook; management there had wrongly heard that Vanderbilt lived. The Daily Racing Form reported that Whitney, knowing the truth, considered scratching Regret, but friends persuaded him to let her run. Out of respect for his brother-in-law’s memory, though, Whitney did not race any more horses that season, but leased his stable out for the rest of the year.

When she retired, Regret had won nine times out of 11 starts. She became a broodmare, but none of her get was as fast as she. She died at 22 in 1934 and was buried on the Whitneys’ farm. The curved headstone above her grave lists her name, breeding, birth and death dates. Then it says, “Winner Kentucky Derby.”