animal black Diceros bicornis horn mammal power rhinoceros vertebrate.danger ear head natural nature nostril rhino snout zoo african black-rhinoceros collagen creases critically-endangered eating endangered folds food gray grazer grey heavy herbivore herbivorous herbivorous-browser hide hook-lipped-rhinoceros horns keratin large lattice-structured megafauna odd-toed-ungulate offspring perissodactyl placid powerful skin southern-white-rhinoceros square-lipped rhinoceros strong surface texture textures thick threatened tough wrinkled wrinkles