A horse died of an apparent heart attack after a Calgary Stampede chuckwagon race Friday night. (CBC)

An outrider horse at the Calgary Stampede has died of an apparent heart attack, becoming the fourth animal to perish at the event this year.

Doug Fraser, a spokesperson for the Stampede, said the horse was cooling down after a race Friday night when it collapsed and died.

A veterinarian determined that the horse, from the Ray Mitsuing chuckwagon team, died of a heart attack.

It's the fourth animal to die this year at the Stampede, and the third chuckwagon horse.

On Thursday, a steer had to be put down after it suffered a spinal injury during the steer-wrestling competition.

Death sparks protest

Animal rights activists from Calgary held a protest Saturday, saying the death highlights how the Stampede is a "cruel spectacle of animal abuse."

The Calgary Animal Rights Meetup Group staged a demonstration outside the Stampede grounds at the Victoria Park C-Train station.

In a written statement, the group said the death toll at this year's Stampede is evidence that all rodeo events need to go.

The group said it would continue to stage protests in order to let the world know "what an embarrassment this spectacle is to our city."

With files from The Canadian Press