If uncastrated, a stallion. If castrated, a gelding. Did you mean "what is a male horse called?" If so, then a male horse who has not been castrated is called a Stallion and one who has been castrated is called a Gelding.
a colt is a young male horse (usually less than the age of 4) a gelding is a male horse that has been castrated and therefore cannot breed a stallion is a male horse that has not been castrated and can therefore breed
Castrated farm animals have different names depending on the animal. A castrated male cow is called a steer. A female who has never given birth is a heifer. A castrated male horse is a gelding. A female spayed horse is still called a mare. A castrated male pig is a hog.