The Blood Horse:

The Kentucky Horse Racing Authority has approved the use of the Vet-Aired Cornell Collar at the state's pari-mutuel racetracks. The throat-support device developed in 2002 at Cornell University is designed to prevent the breathing obstruction known as palate displacement.
The KHRA approved the collar's use after discussions with stewards, state veterinarians, and private equine practitioners.

Under Kentucky regulation, trainers of horses racing with the Cornell Collar for the first time must lodge an equipment change with the racing office and submit a letter from a licensed veterinarian to the stewards stating that the use of the Cornell Collar is justified and appropriate for use on the given horse. Once a horse races with the Cornell Collar it must continue to race with it unless the attending veterinarian submits a letter to the stewards stating that the use of the device is no longer necessary.

The collar isn't allowed for racing purposes in New York or Pennsylvania, but Calder Race Course in Florida does allow it.