The outrider horse collapsed at the end of Heat 9 of Monday's chuckwagon race. (CBC)

A horse was euthanized Monday night after suffering an apparent shoulder injury in a chuckwagon race on the fourth day of competition at the Calgary Stampede.

A veterinarian decided to put the outrider horse down after the animal collapsed following the race, officials said. The animal was part of veteran driver Kelly Sutherland's team.

"The track was in good condition, but that doesn't stop a horse from having a misstep," Stampede spokesman Doug Fraser said.

It was the third horse to die on Monday alone, bringing to four the number of horses that have died since the Stampede began on Friday.

"It is horrific, and the fact that we still have quite a few days left [of the Stampede], it really makes me sad because there could be quite a few more deaths," said Heather Anderson of the Daisy Foundation, a Calgary animal rights group.

Fraser said the Stampede works closely with the Calgary Humane Society and the Alberta SPCA to ensure animals are treated well.

The horse that was euthanized Monday night was part of veteran driver Kelly Sutherland's team. (CBC)

"We've had four incidents that are totally unrelated. It's a concern, but of course we believe we have the best animal care in the industry," Fraser said.

On Monday morning, a chuckwagon horse died of what's thought to have been a heart attack during a training session.

Later that afternoon, a bucking horse broke its back when it bucked after it had thrown off its rider — what Stampede officials call an "empty buck." The animal was euthanized.

On Sunday night, a horse had a suspected heart attack during the team cattle penning competition and fell on its rider, sending her to hospital. Amy Carver suffered a serious head injury and a broken shoulder blade.