The following are the terms used to describe the gender of a horse or pony.

Filly - A female horse or pony not yet full grown.

Colt - A male horse or pony not yet full grown or gelded (castrated).

Mare - A fully grown female horse or pony.

Stallion - A fully grown male horse or pony that has not been gelded (castrated). Stallions are also known as "Entire".

Gelding - A male horse or pony that has been castrated.

Rig - A male horse or pony who has either one or both testes undescended and retained, ie retained in the abdomen.