Why do you say herbivorous animals are the same? There seem to be many different kinds of herbivorous animals. Wikipedia's article on herbivorous animals lists mammals, birds, reptiles, and invertebrates. Among the mammals, they list horses, rodents, panda bears, and of course there are also sheep and goats. Do you think the excreta would be different for panda bears eating bamboo, goats eating grass, and birds eating seeds? The excreta are different because the animals eat different food.