with selective breeding (which doesn't happen so much with humans anymore as pretty much anyone can get laid) that is prevalent in the animal kingdom (only the strongest get to reproduce) the strongest traits of the species are passed on and mutate over thousands and thousands of years (many many more than 6000 I assure you) The species can diverge to create 1 or more different species, some of those die off, some carry on until you're left with a planet that has almost 10,000 different species of mammal. Lumping things together into "horsekind" and "feline" is horribly narrow minded. Yes, Zebra's, Horses, Donkeys, Mules etc all had a common ancestor, yes Lions, Tigers, Cheetahs, Leopards had a common ancestor, but just lumping them into a group like that is wrong. Plainly and demonstrably wrong. And it's the same as saying Chimps, Gorrilas, Apes, Baboons and Humans all had a common ancestor, also demonstrably and provably correct. We have the evidence, you lose.