In the United States, they are primarily intended for animals competing in International Federation for Equestrian Sports (FEI) events.
In the United Kingdom, it is now required for all equines to have a "passport" and an animal cannot be sold without one.
[1] The UK law, passed in 2003, allows owners to keep a horse from entering the food chain for slaughter by signing a declaration which allows the horse to be treated with medications that are inexpensive but not appropriate for animals that will be used as food.
[2] All equines, including horses, ponies, donkeys and other equids, must have a passport and owners can be fined up to £5,000 if the animal does not.
The passport must be renewed every four years and updated if the animal changes ownership.