A few horses have lived into their 40s, and, occasionally, beyond. The oldest verifiable record was "Old Billy," a horse that lived in the 19th century to the age of 62. Sugar Puff, who had been listed in the Guinness Book of World Records as the world's oldest then-living pony, died at age 56.

The following terminology is used to describe horses of various ages:

• Foal: a horse of either sex less than one year old. A nursing foal is sometimes called a suckling and a foal that has been weaned is called a weanling. Most domesticated foals are weaned at 4-6 months of age.

• Yearling: a horse of either sex that is between one and two years old.

• Colt: a male horse under the age of four. A common terminology error is to call any young horse a colt, when the term actually only refers to young male horses.

• Filly: a female horse under the age of four.

• Mare: a female horse four years old and older.

• Stallion: a non-castrated male horse four years old and older.

• Gelding: a castrated male horse of any age, though for convenience sake, many people also refer to a young gelding under the age of four as a "colt."

In horse racing, the definitions of colt, filly, mare, and stallion or horse may differ from those given above. In the United Kingdom, Thoroughbred horse racing defines a colt as a male horse less than five years old, and a filly as a female horse less than five years old. In the USA, both Thoroughbred racing and harness racing defines colts and fillies as four years old and younger.

The English-speaking world measures the height of horses in hands, abbreviated "h" or "hh," and is measured at the highest point of an animal's withers. One hand is 4 inches (10 cm). Intermediate heights are defined by hands and inches, rounding to the lower measurement in hands, followed by a decimal point and the number of additional inches between 1 and 3. Thus a horse described as 15.2 hh tall, means it is 15 hands, 2 inches, or 62 inches (160 cm) in height.

The size of horses varies by breed, but can also be influenced by nutrition. The general rule for cutoff in height between what is considered a horse and a pony at maturity is 14.2 hands (58 inches (150 cm)) as measured at the withers. An animal 14.2h or over is usually considered a horse and one less than 14.2h is a pony. However, there are exceptions to the general rule. Some smaller horse breeds who typically produce individual horses both under and over 14.2h are considered "horses" regardless of height. Likewise, some pony breeds, such as the Welsh pony, share some features of horses and individual animals may occasionally mature at over 14.2h, but are still considered ponies.

The difference between a horse and pony is not simply a height difference, but also a difference in appearance. There are noticeable differences in conformation and temperament. Ponies often exhibit thicker manes, tails and overall coat. They also have proportionally shorter legs, wider barrels, heavier bone, shorter and thicker necks, and short heads with broad foreheads. They often have calmer temperaments than horses and also a high level of equine intelligence that may or may not be used to cooperate with human handlers.

It is thought that the largest horse in (recorded) history was a Shire horse named Sampson, who lived during the late 1800s. He stood 21.2½ hands high (86.5 inches (220 cm)), and his peak weight was estimated at 3,360 pounds. The current record holder for the world's smallest horse is Thumbelina, a fully mature miniature horse affected by dwarfism. She is 17 inches (43 cm) tall and weighs 60 pounds.