Sometimes, for genetic and perhaps environmental reasons, a stallion or stud colt will lack, or never drop, a second testicle. This is considered a major flaw for a stallion and stallions with only one testicle should not be bred. The only way to tell if a horse has only one testicle is to palpate- or feel- the area. First see if you can tell by visually inspecting the testicles. If only one testicle has dropped, a vet can examine to see if the other testicle is missing, or sthe horse simply has not yet matured enough to have dropped both.