What is an adult female domestic horse called?

An adult female domestic horse is called a mare.

The term mare generally applies once a female horse reaches adulthood, although horse terminology can vary slightly by context. A young female horse is usually called a filly, commonly until she is about four years old. A mare can reproduce, but the word describes age and sex rather than breeding history.

Mares carry foals for about 11 months, with an average gestation of roughly 340 days. Their offspring are called foals during the first year of life. A male horse is a stallion when intact and a gelding after castration.

“Mare” is specific to horses and closely related equids in ordinary English usage. A female donkey is commonly called a jenny or jennet, while a female mule is also often called a mare, which can cause confusion when the animal’s species is not stated.