As the debate rages on for Horse of the Year, I suppose Zenyatta had a shot at the Sportsman of the Year (won by Drew Brees) awarded by Sports Illustrated every year. She certainly qualifies as an athlete who has had success in her sport and had a large effect on the sport beyond her winning. And yes, you don't have to be a man to win the title. Chris Evert won in 1976 and Mary Decker in 1983. Teams have also won, the last being the Boston Red Sox in 2004.
No horse has ever won the title, but Jockey Steve Cauthen won in 1977. I went back and read the article on Steve Cauthen by legendary writer Frank Deford. The first thing I noticed is how journalism has changed from when the article was written 33 years ago, back when magazines were filled with ads and readers had attention spans that would read behind 140 characters.
This years article in SI on Brees is a mere 11 paragraphs and 666 words. The article by Deford on Cauthen is a novel by current standards at 62 paragraphs and 4,332 words. You can read the entire article here.