I think because they don't have the same genes or the same arrangement of nitrogen bases in their chromosomes.
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.
The horse. A centaur, a Greek mythical creature, has a human's torso and a horse's lower body.
centaur
fish
There is no fantasy creature that is half horse half human and NOT called a centaur.
If it is a cross between a Human and a: Horse=Centaur, Fish=Mermaid, Goat=Satyr
I think you might be thinking of a hippocamp. http://en.wikipedia.org/wiki/Hippocamp
The breeding of a zebra stallion and a horse mare results in a good percentage of live births. The breeding of a horse stallion and a zebra mare rarely results in a live foal. This is similar to the production of mules as opposed to hinnys when horses are bred to donkeys. The use of a donkey on a mare, (to produce a mule) is much more fertile than the use of a horse stallion on a donkey jenny, (to produce a hinny).
when you become a centaur :L
half man half horse