Each of them have stellar credentials in trying to become the first female to win the year-end honour since Azeri in 2002.
Rachel Alexandra went 8-for-8 this year, including three wins over males, and became the first filly in 85 years to beat the boys in the Preakness Stakes.
Zenyatta improved to 14-0 for her career with a one-length victory against a loaded field of males in the Breeders' Cup Classic, many of whom Rachel Alexandra had already beaten.
The five-year-old mare went 5-for-5 this year, but Saturday's race was her first against the boys. It was also likely her last. Trainer John Shirreffs said Sunday that Zenyatta would be sent to Kentucky to breed, even though Rachel Alexandra is expected to campaign next year.
So who rules in this Year of the Girl?
"You tell me," Shirreffs said.
It's a debate sure to simmer until January, when the Eclipse Awards -- the Oscars of horse racing -- are presented in Beverly Hills. Members of the National Turf Writers Association, the Daily Racing Form and racing secretaries at the major tracks decide the year's champion horses.
Even before Zenyatta's come-from-behind win Saturday, Hall of Fame trainer Bob Baffert suggested the horses should share the top honour.
"If they don't reward her with Horse of the Year, it would be a travesty, or at least co-Horses of the Year," said Baffert, who trained sixth-place Classic finisher Richard's Kid. "It was the only time in horse racing that I didn't mind getting beat in a big race."
Chip Woolley Jr., trainer of Kentucky Derby winner and former Canadian two-year-old champion Mine That Bird, said before the Classic that Rachel Alexandra had already clinched Horse of the Year.
"She's put up eight races on eight different racetracks, beat the boys three times and beat the fillies," said Woolley, whose colt finished ninth in the Classic behind Zenyatta.
"The one thing everybody keeps forgetting, she beat the fillies with authority," he said of Rachel Alexandra. "She didn't win by a neck or a length or two lengths. She beat 'em by 20 and in a big gallop. When you look at that and you put everything together, I think she's a very deserving champion."
Belmont Stakes winner Summer Bird was beaten by Rachel Alexandra in the Haskell Invitational and by Zenyatta in the Classic. His trainer, Tim Ice, was critical of Rachel's absence Saturday at Santa Anita.
-- The Associated Press