Just as Rico suspected

The Associated Press has a follow-up article about those horses:

The sudden death of twenty-one polo horses in Florida may have been caused by a toxin that has yet to be identified by tests and could have been in the animals' feed, vitamins, or supplements, veterinarians said Monday. The horses from the Venezuelan-owned team Lechuza Caracas sickened just before a tournament Sunday, collapsing and dying on the scene, or while being treated at vet clinics or transported, officials said.
Dr. Scott Swerdlin, a veterinarian at Palm Beach Equine Clinic near the polo grounds, treated one of the sick horses. He said it appeared the animals died of heart failure caused by some kind of toxin that could have been in tainted food, vitamins, or supplements, or some combination of all three that caused a toxic reaction. "A combination of something with an error in something that was given to these horses caused this toxic reaction," Swerdlin told reporters. It may take days or weeks to get the results of toxicology tests that could identify the toxin, he said. Another veterinarian who was at the scene said something triggered heart failure among the horses. "Well, clearly, it's an intoxication, clearly there's some sort of a poison," Dr. James Belden told NBC's Today Show.
The team is owned by Venezuelan businessman Victor Vargas but most of the horses and players are Argentine, Swerdlin said. The team travels most of the year. Swerdlin said the team has up to sixty horses. All of those who fell sick have died, he said.
The Lechuza Caracas horses were being unloaded from their trailers Sunday afternoon when two collapsed and others acted dizzy and disoriented, according to the International Polo Club Palm Beach. Seven horses died at the scene and the rest while being treated elsewhere or en route to medical care. The polo grounds in Wellington, a wealthy equestrian and golfing community in central Palm Beach County, hosts the U.S. Open every year.
John Wash, the polo club's president of club operations, told reporters Monday that doctors had ruled out any sort of airborne infection. "This was an isolated incident involving that one team," Wash said. "This was devastating," he said. "It was heartbreaking to see that many horses to get sick all at once." Veterinarians already at the event quickly tried treating the horses, inserting intravenous lines, and trying to cool them down with fans and water. Observers hung blue tarps to shield the horses from the crowd's view.
The match in the U.S. Open Polo Championship was postponed and an exhibition game with a substitute team was held in its place. The carcasses of at least fourteen horses were taken to a state agricultural laboratory for necropsies to learn the causes of their deaths. Wash said these types of horses can be worth from $10,000 to $200,000 each. He also said that because doctors had ruled out an infection, the games will resume Wednesday.