Instructions
Check underneath the horse toward the hind legs. If the horse is a stallion, you will find external genitalia ventrally, or beneath, and between the hind legs. The stallion's external genitalia include the penis, which is located within a prepuce or sheath; and the scrotum, which houses the testes. The penis is commonly drawn upward into the sheath, so is not visible until it descends. When extended, the penis is approximately 50 centimeters long and 2.5 to 6 centimeters in diameter in an adult horse. The oval-shaped testicles are approximately eight to 12 centimeters in length.
Standing at the horse's hindquarters, carefully lift the tail. If the horse is a mare, you will see the labia--the lips of the vulva on both sides of a 12-15 centimeter vertical slit, ventral to and below the anus and underneath the tail.
Look underneath the horse toward the hind legs to check for mammary glands, or teats, which also indicate the horse is a mare. The teats are located ventrally between the hind legs and are smaller in young mares or those that have never been bred. Each teat has two ducts that open externally.
How to Tell If a Horse Is a Mare or a Stallion
A mare is an adult female horse, and a stallion is an adult male horse. According to veterinarian Janet Roark of Hill Country Mobile Veterinary Service in Austin, Texas, the majority of the reproductive system of the mare is internal; and of the stallion, external. As you are checking for the sex of horses, be sure to stand safely to the side of the horse, and do not surprise or startle it. You may have a more difficult time determining the sex of a very young horse.