Bob Baffert-trained Havnameltdown was euthanized on the track after suffering a left front leg injury and unseating jockey Luis Saez in a race on the Preakness Stakes undercard at Pimlico on Saturday.
Saez was taken to hospital complaining of lower leg pain, the Baltimore Sun reported.
The breakdown before the second jewel in US flat racing's Triple Crown at the Baltimore track comes after the Kentucky Derby was clouded by a spate of horse deaths in the build up to the first Triple Crown race at Churchill Downs earlier this month.
Havnameltdown went down coming around the final turn on the inside.
It's the first time since 2016 that a horse has been euthanized at Pimlico on Preakness Day.
"We are just devastated," Baffert said in a statement. "This is a shock to everyone at our barn who love and care for these horses every day.
"Hanvameltdown was obviously hit pretty hard coming out of the gate. We don't know if that contributed to the injury, but we will be fully transparent with those reviewing this terrible accident.
"Right now, our thoughts are with Luis Saez and we are hopeful he will be okay."
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