Some animal species have become extinct because they were less successful than other species, which gradually replaced them. But the number of rhinoceros has reduce due to hunting.
Poaching or illegal hunting has reduced drastically the number of black rhinos. Most of them are surviving and are conserved in national parks and wildlife sanctuaries.
The rhino is hunted specially for its horn. The rhino horn can grow up to 62 inches and fetches a good price when sold, thus making the animal endangered.
Below Rhinoceros info from Wikipedia (Read More):-
Rhinoceros (pronounced /raɪˈnɒsərəs/), often colloquially abbreviated rhino, is a name used to group five extant species of odd-toed ungulates in the family Rhinocerotidae. Two of these species are native to Africa and three to southern Asia. Three of the five species—the Javan, Sumatran and Black Rhinoceros—are critically endangered. The Indian is endangered, with fewer than 2,700 individuals remaining in the wild. The White is registered as "vulnerable", with approximately 17,500 remaining in the wild, as reported by the International Rhino Foundation.[1][2] The Rhinoceros Beetle is named as such due to its horns, which simulate rhino's horns.
http://en.wikipedia.org/wiki/Rhinoceros
