Pygmy elephants and orang-utans are both at risk in Borneo as the forests they live in are being cut down.
The WWF report that in 15 years' time less than a third of the forest will be left, especially in the parts of the island where the most animals live.
The loss of forest in those areas is so bad that it could disappear completely in 10 years' time.
The WWF's Tessa Robertson said: "The loss of Borneo's forests is a disaster for both species and people."
The loss of the forests is called deforestation, and there are three main reasons why it is happening.
Illegally cutting down trees for construction, called loggingConverting the forest to land for growing crops, like palm oilForest fires
Robertson added that the forests were being turned into plantations to provide people living there with money now, without properly considering what it would mean in the future.
Things did take a turn for the better recently though, when one of the places where trees illegally cut down were transported was closed.
More than 210 mammals, including 44 which are found nowhere else in the world, live on Borneo.