I think because they don't have the same genes or the same arrangement of nitrogen bases in their chromosomes.
A centaur. LOL
an organism which is the result of mating between the two species of different genera example; mating between donkey and horse create mules
A creature with the body of a horse and torso of a person is a centaur.
It is called a centaur. If the upper half is human with the lower half horse and the horse section has all four horse legs, it is indeed a centaur. If it has the upper half of a human with the lower half of a horse but only 2 legs like a satyr it is a Sileni. There are also horse-headed men but I can not find the names for them. There is also the deity Hayagriva in Hinduism and Buddhism that has a white horse's head and a human body and is pictured as seated on a lotus.
centaur
fish
There is no fantasy creature that is half horse half human and NOT called a centaur.
A half human half horse is a centaur, a Greek mythical creature.
I think you might be thinking of a hippocamp. http://en.wikipedia.org/wiki/Hippocamp
The horse. A centaur, a Greek mythical creature, has a human's torso and a horse's lower body.
when you become a centaur :L
half man half horse
A centaur