Mules would never happen in the wild. It's entirely a creation caused by man (anthropogenic). It has to do with animal stress being in captivity and not in their natural habitat. Even domestic animals are really animals bred with a Down's Syndrome like characteristic. Wolves are far more intelligent than their domestic cousin the dog. Domestic pigs are far inferior to the intelligence of a feral pig. There are also very distinct physical traits between domestic and wild animals too. When they talk about hybridity at zoos and other controlled environments aberrations like the liger/tigon, onager (half horse, half zebra) and others are produced as an attraction for visitors. It's a business selling product. In reality, such an occurence would never happen in Nature. Even if it did, the aberration would be eliminated by one mechanism or another. One of Nature's immutable laws is the preservation of species. The strictness of this cannot be underestimated. There are over 30 sub-species of White-tailed deer. Their ranges and habitat greatly overlap. Yet, the preservation of each subspecies stays in fully in tact.