King Fury, the 20-1 Kentucky Derby entrant named for world heavyweight boxing champion Tyson Fury, was scratched from Saturday's US racing classic after spiking a fever on Friday.
Kenny McPeek, King Fury's trainer, said in a Twitter posting the horse had an elevated white blood cell count and in an attached video said the horse was not eating.
"King Fury is going to be scratched for tomorrow @KentuckyDerby," McPeek wrote. "Spiked a high fever this afternoon & white count elevated. I'm disappointed for all those involved, obviously the right thing."
The chestnut colt was named for boxer Fury, the British fighter who holds the World Boxing Council heavyweight crown.
"King Fury spiked a 104 (F) fever this afternoon after he galloped this morning, went off his feed. We obviously have to do the right thing. I feel gutted," McPeek said. "Unfortunately, he's not 100%.
"This is a horse that had been doing really well all week. Anybody that watched him train had to be impressed. I know I was and I felt pretty confident going into this race that we had a big chance.
"We're obviously going to scratch him on veterinary advice, which is an easy one, take him out.
"We'll rebound and keep at it. King Fury will be fine. Unfortunately he's not going to be able to run."
The Derby at Churchill Downs in Louisville, Kentucky, is the first of the US flat racing Triple Crown series that also includes the Preakness in two weeks at Pimlico in Baltimore and June's Belmont Stakes in New York.
McPeek said he would likely plan on King Fury running again at the Travers Stakes in August at Saratoga.
"We'll regroup and point him towards another race," McPeek said. "I think you'll probably see this horse pointed toward the Travers, which I think is a great spot."