It can be slightly difficult to tell a horse's sex at first glance, but male and female horses have some fairly marked differences that are easy to identify once you know what to look for.
Identify The Sex of a Horse
Stallion: A stallion is an intact adult male horse who possesses all the physical equipment needed to reproduce. His genitals can be located fairly easily even by individuals who have not spent a lot of time working with equines. A quick glance at the pelvic area on the horse's underside between his belly and his hind legs will reveal both his penis and testes. These will be clearly visible, especially if the horse's penis is not retracted. The penis is still visible even when it is retracted into the sheath.
Gelding: A gelding is a male horse who cannot reproduce because he has been gelded, a surgical procedure that removes the testicles and prevents unwanted breeding. If you look underneath an adult horse and see a penis but no testes, you are most likely looking at a gelding.
Rarely a stallion will have one or both testes fail to drop. In this case, a veterinarian will have to use X-rays to determine the location of the missing testes and geld the horse.
Mare: A mare is an adult female horse. If you look at her underside, you may see her udder. If her tail is lifted, you will see that she has two openings: the anus and the vulva. The vulva is part of the birth canal.
Young horses: Fillies, which are young female horses, are not significantly sexually different than adult mares. Colts, which are young males, will have a penis but the testes may not be visible if the colt has just been born.