A colt is a young male

Male

Male refers to the biological sex of an organism, or part of an organism, which produces small mobile gametes, called spermatozoa. Each spermatozoon can fuse with a larger female gamete or ovum, in the process of fertilization...

 horse

Horse

The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...

, under the age of four. The term "colt" is often confused with foal

Foal

A foal is an equine, particularly a horse, that is one year old or younger. More specific terms are colt for a male foal and filly for a female foal, but these terms are used until the horse is age three or four. When the foal is nursing from its dam , it may also be called a suckling...

, which refers to a horse of either sex under one year of age.

An adult male horse if left intact is called either a "stallion

Stallion (horse)

A stallion is a male horse that has not been gelded .Stallions will follow the conformation and phenotype of their breed, but within that standard, the presence of hormones such as testosterone may give stallions a thicker, "cresty" neck, as well as a somewhat more muscular physique as compared to...

" or a horse (sometimes full horse); if castrated

Castration

Castration is any action, surgical, chemical, or otherwise, by which a male loses the functions of the testicles or a female loses the functions of the ovaries.-Humans:...

, it is called a gelding

Gelding

A gelding is a castrated horse or other equine such as a donkey or a mule. Castration, and the elimination of hormonally driven behavior associated with a stallion, allows a male horse to be calmer and better-behaved, making the animal quieter, gentler and potentially more suitable as an everyday...

. A rig or ridgling

Ridgling

A ridgling or rig is a male animal with an undescended testicle. An undescended testicle is not a serious or life-threatening condition, though it may cause the animal discomfort at times...

is a horse or colt with a retained testicle or one which has been incompletely castrated.

A young female horse is called a filly

Filly

A filly is a young female horse too young to be called a mare. There are several specific definitions in use.*In most cases filly is a female horse under the age of four years old....

until age four, and a mare

Mare (horse)

A mare is an adult female horse or other equine.In most cases, a mare is a female horse over the age of three, and a filly is a female horse age three and younger. However, in Thoroughbred horse racing, a mare is defined as a female horse more than four years old; in harness racing a mare is a...

thereafter.

In the wild, colts are driven from their herds by the herd stallion somewhere between the age of one and two. This may be in part an instinct to prevent inbreeding

Inbreeding

Inbreeding is the reproduction from the mating of two genetically related parents. Inbreeding results in increased homozygosity, which can increase the chances of offspring being affected by recessive or deleterious traits. This generally leads to a decreased fitness of a population, which is...

. When driven out, they usually join with other young stallions in a bachelor herd

Bachelor herd

A bachelor herd is a gathering of juvenile male animals who are still sexually immature, or of 'harem'-forming animals who have been thrown out of their parent groups but not yet formed a new family group....

. They stay with this band until they are mature enough to form their own herd of mares. The terms "rag" or "rake" have been historically used to refer to a group of colts, but they have fallen out of modern usage.