A yearling is an animal that is one year old. That is the most basic definition. However, in the equine world, a horse is a year old on January 1 in the northern hemisphere and on August 1 in the southern hemisphere. Since most horses are born in the late winter or spring, this rarely means that a horse will be born in December and officially be a yearling when only a month old in January.
A thoroughbred yearling is often ready to be sold at the spring horse auctions. By that time, it is evident whether it will have the proper conformation for a race horse, and the owner and trainer will usually know if it is a sprinter or a distance-runner, or a saddle companion. For a horse to be a thoroughbred, incidentally, it must be able to trace its ancestry back to one of three stallions in England in the late 17th and early 18th century. These were Arabians mated with English mares to create distance runners. A thoroughbred must also be registered with the local jockey club, which keeps track of all bloodlines and winnings.
Some horses are raced in the yearling stage, but most are taken for training at that age. Serious racing usually begins at age two and finds its zenith at races for three-year-olds. The most famous of these races is the Kentucky Derby held each year at Churchill Downs in Louisville, Ky.
A yearling thoroughbred can be a pricey animal. Depending on its conformation and bloodlines, it could be sold for well over US$1 million. Most are sold for about half that, but any animal that can be traced back to a Derby winner, such as Secretariat, Citation, Affirmed or Seattle Slew, is worth a great deal of money. Many horse owners buy a thoroughbred yearling not because they feel it will be a great race horse, but because they know it will improve their farm's bloodlines. A yearling thoroughbred that does not look at all promising for racing may still produce great racers as a breeder, or may just improve the line in general.