Pioneerof the Nile, sire of 2015 U.S. Triple Crown winner, American Pharoah, died at WinStar Farm in Versailles, Ky. on Monday, the farm announced.
On Monday morning, Pioneerof the Nile bred a mare, then began acting uncomfortably upon returning to his stall, the farm said. The horse passed away while en route to a clinic.
Pioneerof the Nile was 13 years old.
"We are all extremely saddened by the loss of Pioneerof the Nile," said WinStar Farm president and CEO Elliott Walden. "He was a superior physical specimen, a Triple Crown sire, and a unique personality. All of us at WinStar are heartbroken."
Zayat Stables owner Ahmed Zayat took to Twitter to express his grief over the loss of the stallion.
In 2009, Pioneerof the Nile finished second at the Kentucky Derby. He won the 2009 Santa Anita Derby, and won over $1.6 million in career earnings.