* Rhinos are in the order Perissodactyla, which includes horses and tapirs. This order is characterized by having odd numbers of toes, and a unique digestive system adapted to eating tough plant material.
* Pigs are in the order Artiodactyla, which includes cows, deer, hippos, and camels. This order is characterized by having even numbers of toes, and a more complex digestive system that allows them to digest a wider range of food sources.
While both rhinos and pigs are herbivores, they evolved their herbivorous diets independently, and their lineage diverged long ago. They are actually more closely related to horses and hippos, respectively, than to each other.