Horses and donkeys can breed and produce sterile offspring known as mules. Horses and donkeys remain separate species because of this hybrid sterility, which is:

Which is a post-zygotic barrier to reproduction. The postzygotic reproductive barrier is a component that lessens the feasibility or regenerative limit of half-breed posterity. Half-breed zygote variation from the norm is a sort of postzygotic hindrance in which cross breed zygotes neglect to develop regularly.