Police said the rider was in the saddle around 6 a.m., when the horse went into an "unexplained wild gallop," showing signs of medical distress.
The rider, who was identified as Mourad Boudraa, 40, was crushed into the track railing and then by the weight of the horse as it collapsed.
Jamie Martin, vice-president of Woodbine Entertainment Group, said Boudraa was originally from Morocco and had three to four years of experience as an exercise rider.
"He was a freelance so he worked for a number of trainers," Martin said.
"I would say it's a freak accident."
The horse, Tawney's Wish, died of what Woodbine Racetrack officials said was believed to be a heart attack. The three-year-old dark filly had one start this year and $368 in winnings.
Mike Pownall, an equine veterinarian, said the horse's cause of death won't be known until a necropsy is performed.
"It's like when you find those athletes that suddenly die and you find there's a heart defect, that would be the human equivalency," Pownall said. "You have no idea it was existing, or you could prevent it, or you could do anything to alter the course."
In 2011, Canadian Olympic champion Eric Lamaze was riding his horse Hickstead in Italy when the animal suddenly collapsed and died of an acute aortic rupture. The horse had carried Lamaze to gold and silver at the Beijing Olympics in show jumping.
The Ontario Racing Commission, which requires a necropsy whenever a racehorse dies, collects information for a database on common racehorse injuries in the hopes of minimizing future problems, Pownall said.
Pownall said working with horses carries inherent risk and those involved in the industry are all too aware of what can go wrong.
"They know the potential for unexplained, sudden things to happen," he said. "It's a hazard of the job."
But in the tight-knit horseracing community in Canada, word travels fast when there is an injury or fatality, he said.
"I was sad for the horse, sad for the rider. Nobody wants that. I'm proud of the riders that go out there every day and proud of the horses that go out there," he said, adding that Woodbine Racetrack is a "well-regarded track" in terms of safety standards.
Officials from the provincial Ministry of Labour are investigating the accident. A necropsy on the horse is set to be performed at the University of Guelph.
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