Horses eat grass, hay & oats; most of their food passes through their systems undigested with the seeds of the plants they eat still intact. When they randomly lift their tales, the undigested seeds are naturally distributed back into the eco-system. Any plant matter that does not bring forth seed, becomes beneficial fertilizer. Dog poop does not contribute as they eat animal by-products & fillers, hence the need to remove it. Secondly, animals poop outside, if it bothers you so much carry a shovel or wear boots.