If confirmed, the animal, which has dark red fur and a long, bushy tail, would be first new carnivore species discovered on the island since 1895, when the Borneo ferret-badger was found, the fund said.
Cameras set up to photograph wildlife in Kayan Mentarang National Park on the Indonesian side of Borneo island have twice captured images of the animal, said Stephan Wulffraat, a Dutch biologist who is coordinating the WWF's research into the species.
"We have consulted several Bornean wildlife experts. Some thought it looked like a lemur, but most were convinced it was a new species of carnivore," Wulffraat said. "Until we have a live specimen in our hands, we can't be 100 percent sure. Now, I'm only 90 percent sure."
Since 1994, researchers have found more than 360 new species on Borneo island, most of them insects and plants.
The island, which is shared by Malaysia, Indonesia and Brunei, has some of the most diverse wildlife on Earth, but its forests are under threat from expanding rubber and oil palm plantations.