A gelding collapsed and died moments after winning a $US150,000 jumps race in Tennessee on Saturday.
Results of a necropsy are expected soon, with a heart attack or aneurysm considered the most like causes.
Arcadius, aged 8, died as he was being cooled down after his win in the 71st running of the 3-mile Iroquois Steeplechase at Percy Warner Park.
It was the horse’s second start of the season in a 21-race career that began in 2008, with his final race win taking his total race earnings over $US300,000.
His last win was in the feature race of the day, before 25,000 spectators.
The horse showed no signs of distress during the race, which was run in good conditions.
A veterinarian was there to help Arcadius moments after he went down, but efforts to save him were in the vain.