Zebras reflect the morality of every horse and every horse has a corresponding zebra. A horse is born with equal potential for good and evil and over time chooses which to become. A horse that chooses good will shed it's evil and the corresponding zebra will be pitch black. A horse seduced by sin will have its goodness live on as a pure white zebra. Long ago, that was the way it had always been, but overtime the creation of pure good and evil beings became too much as both sides would war. The horses realized they had more agency and thus chose to impose a life of average goodness on themselves. No more creating truly evil zebras to be pure good horses, and the wicked chose to curb their descendants to limit the wholly good zebras. So, zebras have stripes to represent horse culture's rules of balancing good and bad in oneself to prevent civil war among equinekind.