Donkeys come in different breeds and colors, just like horses do, so some of the details of conformation can vary a bit (and just like with horses, there is good and bad conformation). The biggest giveaway is the tail - a donkey has a tail that is a bit like a cow's tail; the long hairs are only on the end. The bony part of a horse's tail is shorter than a donkey's, but the longer hairs go all the way to the body. Mules and hinnies have conformation that is somewhere between that of a donkey and a horse, but they basically have horse tails.