There are five different species of rhinoceros, two in Africa, and three in Asia.
The Sumatran rhinoceros is the smallest of all rhinoceros species. Unlike other Asian rhinos, it has two horns, rather than one. It is also unique among rhinos in being covered in hair.
There are two subspecies of Sumatran rhino, the Western Sumatran rhinoceros, which is the species found in Sumatra, and the Eastern Sumatran rhinoceros, which is found in Kalimantan (although this population is doomed, as it numbers only around 10 individuals). In the past, the Sumatran rhino had a much larger range across Southeast Asia, however these other populations are now extinct.
The Western Sumatran rhinoceros lives in three national parks in Sumatra. These are Way Kambas National Park, and Bukit Barisan Selatan National Park in southern Sumatra, and Gunung Leuser National Park in North Sumatra. The total population is unknown, but is estimated at around 100 individuals, a number which has shrunk by 80% since 1985.
Rhinos are believed to inhabit only a small part of the Gunung Leuser National Park. They require a specifically dry forest habitat, a habitat which is rare in Sumatra. However, as the Gunung Leuser National Park is large relative to Way Kambas and Bukit Barisan Selatan, the total rhino habitat is largest in the Gunung Leuser National Park. Sumatran rhinos seek rivers, and avoid areas with roads.
Poaching is a major threat to rhinos, as their horns are worth around $10,000/kg. In addition, deforestation has substantially reduced the habitat of the Sumatran rhino. They are very large mammals with low population density, and require a large area of undisturbed forest (they have a range of up to 50 square kilometres).
A breeding program for Sumatran rhinos began in the 1980s, but it was spectacularly unsuccessful, as Sumatran rhinos are difficult to breed, and ended up reducing the wild population of the animals.
Sumatran rhinos are solitary animals, but they leave their mark in the jungle by damaging trees, as a form of communication. Although you are unlikely to see a Sumatran rhino near Bukit Lawang as it is not a part of their habitat, there are some marks in the jungle from rhinos having rampaged through in the past.