Over 360 new species have been discovered in Borneo over the last decade, highlighting the great need for conservation in the area, the WWF says.
This brightly coloured insect was discovered on the Sangkulirang Peninsula, East Kallmantan
Previously unseen insects, frogs, fish, lizards and snakes have made themselves known to science for the first time.
And a new report suggests thousands more species remain undiscovered.
However, these newly introduced and yet-to-be-uncovered species are also under threat, WWF claims, because Borneo's forests are being cleared.
"Borneo is undoubtedly one of the most important centres for wildlife in the world," said Tess Robertson, head of the forests programme at WWF-UK. "It is one of the only two places on Earth where orangutans, elephants and rhinos can be found."
Apart from the famous orangutan, Borneo is home to other threatened species such as the clouded leopard, the sun bear and the Bornean gibbon.
Amongst the 361 new species discovered since 1994 are a catfish and a giant cockroach, believed to be the largest in the world.
Other species include 260 insects, 50 plants, 30 freshwater fish, seven frogs, six lizards, five crabs, two snakes and a toad.
WWF's report, Borneo's Lost World, suggests that a panoply of species may yet be found, especially in the largest and most pristine forests in the heart of the island, which is relatively inaccessible.
However, these species, along with their better known compatriots, have an uncertain future because of the timber, rubber, palm oil and paper trades.
Since 1996, deforestation in the whole of Indonesia has increased to an average of two million hectares per year - an area about half the size of the Netherlands.
The WWF claims that logging is set to rise because of the country's growing population and the soaring demands of international markets.
According to the report, the illegal trade in exotic animals is also on the rise, as logging trails and cleared forest open access to more remote areas.
Hydrophis sibauensis is a highly venomous species of water snake, first described in 2001
The WWF says it is working with Brunei, Indonesia and Malaysia on a new initiative to conserve the area known as the "Heart of Borneo" - a total of 220,000 sq km of equatorial rainforest - through a network of protected areas and sustainably managed forest.
"The forests of Borneo are crucial not just for the protection of wildlife but also to safeguard water resources necessary for the prosperity of the island," said Ms Robertson.
"Losing the heart of Borneo would be an unacceptable tragedy not only for Borneo, but for all of Asia, and the rest of the globe. It really is now or never."
A new "glass" catfish called Ompok platyrhynchus