2013 - The forests of Borneo are considered to be the oldest and for this reason the most biodiverse tropical rainforests. The fauna of Borneo has certain peculiarities. So far, 221 species of mammals are known to exist in Borneo. These include primates orangutans (which are otherwise only found in northern Sumatra) and gibbons as well as the proboscis monkey, which only occurs in Borneo.