Just like the human athletes, horses that compete in the Olympic Games have a “citizenship” requirement — they have to be owned by a citizen of the country that they represent at the games for a certain period of time before competing. The deadline for this period of time is laid out by the Federation Equestre Internationale (FEI), which is in charge of the rules for equestrian sports at the Olympics. For the 2012 Olympics, any nationality changes for the horses must be complete 30 days before the event in which they are going to compete.
More about Olympic horses:
The process for officially changing a horse’s nationality to be able to legally compete takes time. The nation that wants to apply for the horse’s nationality change has to write a letter to the FEI formally requesting the change. The letter must be signed by the president or secretary general of the nation, along with the reason for the change and details about the person who will be riding the horse, among other things.
The Olympics have featured equestrian events continuously since the 1912 Summer Olympics. The first equestrian Olympic events were held in 1900.
Horses competing in the Olympics must be at least seven years old and must pass drug tests to be able to compete.