After seven deaths raised questions about the future of horse racing, Mage earned a surprising Kentucky Derby victory, capping a nerve-rattling day that included two more fatalities ahead of the 149th edition of the famous race.

Mage, a 15-1 shot, had only one win in his career, giving little indication that he could triumph against 17 rivals in a race that is not kind to the inexperienced.

Still, he made a gutsy stretch run, overtaking Two Phil's to his inside and winning by a length. Mage, who didn't race as a two-year-old, ran the 1 1/4 miles in 2:01.57 at Churchill Downs on Saturday.

Mage joined Justify (2018), Big Brown (2008) and Regret (2015) as Derby winners with just three previous starts.

"He proved today that it didn't matter," assistant trainer and co-owner Gustavo Delgado Jr. said.

The colt was in the skilled hands of Javier Castellano, a Hall of Fame jockey who was successful in his 16th start in the arce.

"I never give up," Castellano said. "I always try hard to do the right thing. It took me a little while to get there. I finally got it."

Castellano and trainer Gustavo Delgado are from Venezuela.

"Everything went according to plan," Delgado Jr. said. "This is the dream I have, a year-and-a-half ago, I wrote a note: 'We're going to win the Derby next year.'"

Forte, the early favourite, was scratched in the morning with a bruised foot, one of five horses that dropped out in the run-up to the race.

Mage earned $US1,860,000 ($A2.75m) for the biggest win of his brief career.

Earlier in the day, Chloe's Dream, a three-year-old gelding, and Freezing Point, a three-year-old colt, were euthanised after being injured in their races, becoming the sixth and seventh horses to have died at the track in recent days.

"It's a very difficult subject to touch upon," said Ramiro Restrepo, part of Mage's ownership and a blood stock agent. "I'm sure there's going to be some investigations done as to the reason behind that, and hopefully that provides a few more answers."

Chloe's Dream injured his right front knee, his trainer Jeff Hiles said.

"He just took a bad step out there," Hiles said. "They could do the same thing running in the field as they could on the track. So it's very unfortunate. That's what we deal with."

Freezing Point suffered a left ankle injury in the Pat Day Mile, trainer Joe Lejzerowicz said.

"He just got bumped in the backstretch," Lejzerowicz said. "He never took a bad step or bobble. He had a big heart."

New antidoping and medication rules enforced by a central governing body of the sport are scheduled to take effect on May 22.

The deaths included Derby contender Wild On Ice. Two of the horses were trained by Saffie Joseph Jr. He was indefinitely suspended by the track, although investigators have yet to determine a cause for the deaths of his horses.