Ceratotherium simum (white rhinoceros)
Dicerorhinus sumatrensis (Sumatran rhinoceros)
Diceros bicornis (black rhinoceros)
Rhinoceros sondaicus (Javan rhinoceros)
Rhinoceros unicoris (Indian rhinoceros)
The word used to describe this huge but docile animal, rhinoceros, comes from the Greek words for "nose" (rhino) and "horn" (ceros). There's a lot more to a rhino than its horn, which usually grows to an average length of 30-45cm, though it is clearly the most distinguishing feature on just about all rhinos.
Rhinos are loosely related to horses, and more closely related to tapirs, with whom they share their characteristic thick, leathery skin, their prehensile lips, and their tendency to spray very powerful streams of urine at predators or nuisances when in distress. (It seems Webby's info is, not surprisingly, out of date.) Humans are the only creatures above the rhino on the food chain, although various parasites and stinging insects make their homes in and on rhinos. In the wild, they usually only fight with each other. A bull rhino must fight for dominance to win breeding rights -- otherwise they're pretty laid-back creatures. In adulthood, the horn may look frightening, but it's usually used only for digging holes in the ground, gouging at particularly thick plantlife (all rhinos are herbivors), and as a kind of sensor, which the rhino tends to rub against any and all things it encounters. Older rhinos are sometimes hornless, due to excessive rubbing throughout its life, where the horn breaks off or is worn down to a nub. Baby rhinos are born without horns, but they start to develop soon after birth and are usually at full length by the time they reach sexual maturity within seven to eight years (males), or five to six years (females).
Rhinos can usually be found wandering leisurely from grassland to grassland, eating their fill and moving on. During the summer, they enjoy wallowing in mud pits to keep cool, and to keep biting insects from bothering them. Their thick, armour-like skin is actually quite sensitive, and is not immune to sunburn or most insects. When not wallowing in the mud, a few species of small birds will perch on the rhino's back, eating the insects it finds. The rhino tolerates the presence of the birds in exchange for their help with combatting the insects.
Most rhinos grow up to be between 1.2m and 2m tall at the shoulder, and weigh between 800kg and 2,300kg, depending on species. The white rhino is the largest, and, along with the remaining Indian rhinos, is the most plentiful species and the most likely to be found in the wild. The Sumatran rhino, diminutive by comparison, is the smallest, and is also the animal that early explorer Marco Polo mistook for a unicorn upon his first visit to Java.
As you may have guessed by the names, the Indian, Sumatran, and Javan rhinos are all native to various Asian countries -- Indian rhinos could once be found all over India and Nepal, and Sumatran and Javan rhinos inhabited Vietnam, Malaysia, and Indonesia. The black rhino (which is actually grey) and the white rhino (also grey, but typically a lighter shade than the black rhino) are natives of a sizeable number of African countries, with the largest populations in the eastern and southern countries.
The habitat of the various species of rhino varies, covering large areas of Africa and smaller but denser areas on the Indian subcontinent. All five species are currently endangered, with three of the five considered to be at a critical risk of extinction. One particular species, the Mongolian rhino (Baluchitherium grangeri), an animal that was larger than the modern elephant and considered to be the largest land mammal to ever exist, died out several million years ago, most likely due to changes in climate. Currently, there are approximately 11,000 animals left in the world, though the white rhino has been undergoing a captive breeding program in various zoos since the 1970s, and is making a slow but steady comeback. Only about 60 Javan rhinos are alive right now, as most of them have been slaughtered by humans for their horns, which, in their part of the world, is a very popular (and likely useless) aphrodesiac. Sumatran rhinos have dwindled severly, as well, for the same reason, and now only about 200 are left, though they also are undergoing captive breeding. The black rhino numbers at about 2,000. Like elephants being poached for their tusks, rhinos are (or were, but probably still are) killed, de-horned, and then left to rot. Most black and white rhinos had their horns turned into dagger and sword handles in Middle Eastern countries, where such a thing is a symbol of status and wealth. (Gee, what a great idea.)
Rhinos are actually rather friendly creatures, as long as you don't seem threatening or try to approach their young. Those held in zoos are frequently quite docile, allowing their handlers to clean them and feed them. However, if you encounter one in the wild, you probably won't survive the encounter, if young rhinos are nearby. If a rhino feels its young is in danger, it will charge, and as it can gallup at a top speed of about 60 km/h, it'd be able to gore you with its horn before you got even a few meters away. Hopefully, only poachers will ever incur its wrath, but then, poachers never seem to go away. I can understand killing an animal for food, in moderation, but killing an animal for a single part of its body and then leaving the rest of it to rot has got to be one of the few truly evil things that humans can still do that doesn't involve killing other humans. Most of the other evil things have become part of the faceless corporate and governmental machine. Nevertheless, with any luck, the captive breeding programs will continue to succeed and the rhinoceros populations will continue to grow instead of decrease because of needless causes.
Update: The west African black rhinoceros was declared extinct in the wild on November 10, 2011.