Having hooves is a characteristic of ungulates, which are mammals that have hooves instead of claws or nails. Ungulates are divided into two groups: odd-toed ungulates, such as horses, donkeys, and rhinoceroses, and even-toed ungulates, such as cows, sheep, and pigs. All ungulates have hooves, which are made of keratin, the same material that makes up human fingernails. Hooves are strong and durable, allowing ungulates to walk on rough terrain and protect their feet from sharp objects.