Tiffany Foster's horse Victor was found to have hypersensitivity in the left front leg by competition veterinarians shortly before the individual and team jumping events began Sunday.
"I never imagined when I came to the Olympics that I would be unable to compete because of something like this," said a distraught Foster. "I feel so very badly for my teammates. I never dreamed that this is the way that my first Olympics would end."
Canada's chef d'equipe Terrance (Torchy) Millar said Victor had a minor sore spot on his leg, possibly from knicking himself with a hoof in the stall. An protest lodged by Millar was denied based on International Equestrian Federation regulations, which state there is no grounds for an appeal against the decision to disqualify a horse for "abnormal sensitivity" from an event.
"We are very unhappy about this. It is a decision that lacks any common sense," Millar said.
"It is just blind application of a rule. It lacks judgement and horsemanship. The horse has one small nick on one coronet band. He could have got it anywhere. The horse is sound."
Canada's leadoff rider Eric Lamaze, who won individual gold in Beijing in 2008 while his team took silver, said his teammates are determined to be on the medal podium Monday where Foster can join them.
"That would be the biggest reward of all," Lamaze said. "She has the support of all her teammates."
Foster, from Schomberg, Ont., was tied for 60th following the opening round of individual qualifying Saturday at Greenwich Park.
Her absence puts Canada at a disadvantage. The team is allowed to drop one bad score with four riders, but need to keep every score with three riders. Canada still finished the day sixth in team qualifying to move onto the next round.
The issue of hypersensitivity has followed international show jumping for years, since capsaicin — the main ingredient in chili peppers — can be used to make a horse's legs over-reactive to touch and thus jump higher. Several jumping horses were disqualified at the 2008 Olympics for testing positive to capsaicin.
However, jumpers in competition can easily get small leg cuts, and the tests for hypersensitivity distinguish between heat in a leg caused by a minor sore or by a foreign substance. Millar said Victor was not tested for the presence of a foreign substance.
"I'm sick about it," said Canadian rider Ian Millar, no relation to the chef d'equipe. "The way the rule is written, it's not an equitable interpretation."
Lamaze's training business, Torrey Pines Stable, owns Foster's mount in partnership with Artisan Farms.
"I am ashamed of our sport today," Lamaze said. "This is a complete miscarriage of justice. Yes, the horse has a little, superficial cut on its coronary band that could have happened in any number of ways.
"The horse was ridden in the morning, and was jumped as part of his exercise routine, with no indication whatsoever that he was uncomfortable. The horse was not bothered by it, and we had no doubts that competing would not have caused any further harm."
Princess Haya Al Hussein, president of the FEI, said later at a press conference that there was no suspicion of a foreign substance.
"I want to be very clear that there is no accusation of malpractice," said Haya. "We all need to be protective of the horses."
The FEI president's comments were followed by those from a tearful Foster.
"I just want to say I would never do anything to jeopardize the welfare of my horse," said Foster. "I feel really bad for my team."
Lamaze and Millar also advanced on the second day of the individual competition. Lamaze, also from Schomberg, was tied for 13th with one penalty point for a time violation.
Lamaze was left shaking his head after an error atop Derly Chin de Muze dropped him from the leaders.
"She is a great young horse but she has a big stride. And the time fault was my fault," he said.
Millar, from Perth, Ont., followed in 17th with four penalty points, while Jill Henselwood of Oxford Mills, Ont., just missed out on qualifying in 47th with nine points.
Millar had a clean run and credited his horse Star Power will calming down after the rough start to competition Saturday.
"He was tense yesterday," Millar said. "I love the fans and the crowd and I don't wish to criticize, but they elevated the level of anxiety of my horse.
"Today was perfect. My horse was sharp, up in the air and listening to me, I was very pleased with the step up from yesterday."
Canada came into the events with medal hopes after the team took silver and Lamaze won gold four years ago.
Millar and Henselwood were also part of the 2008 team that finished second with three riders, a feat Canada is now forced to try to duplicate in London.
— With files from the Associated Press.
Also on HuffPost