Horses and donkeys are of different species. In order to get what we call a "mule," a horse and donkey must mate and produce offspring. Any offspring between two different species will be sterile and unable to reproduce.
DID YOU KNOW? Marlon Brando hated memorizing lines so much that he posted cue cards everywhere to help him get through scenes.
He even asked for lines to be written on an actress's posterior. (That request was denied.)