Check Mate falling and injuring herself during a rookie saddle bronc event.
Check Mate was euthanised after her injuries were assessed by a veterinarian.
The organisation, SHowing Animals Respect and Kindness (SHARK), reacted angrily to the death, and released a video of the incident.
SHARK said it was video-documenting the entire rodeo.
The death of Check Mate, it said, followed the death in 2009 of another horse, named Strawberry Fudge, in the rookie saddle bronc event.
SHARK said it had also captured her death on video.
SHARK noted that in the previous four days, four calves and two steers, injured so badly they were unable to either stand or leave the arena on their own, were put on a small truck and removed from the arena.
SHARK president Steve Hindi and associates, who have been at each rodeo performance, issued a statement condemning the treatment of animals at the event, saying it was a shame on the entire country.
"Sponsors and attendees of this cruelty should be ashamed of themselves. The media that whitewashes such abuse don't deserve to be called journalists."
SHARK said the rookie saddle bronc event had a history of horses out of control and taking bad falls.
It labelled animal welfare standards being applied as phony and useless