The photo, released Friday, is the first record of the "lost muntjac of Sumatra" in 80 years, says U.K. conservation group Flora & Fauna International.
An anti-poaching patrol had photographed the mountain deer at 6,400 feet (1,950 meters) in 2002.
It was only recently, however, that muntjac expert Robert Timmins recognized the rain forest deer in the photo as the first documented Sumatran muntjac since 1930.
The species closely resembles the red muntjac. But Timmins was able to identify the Sumatran in the photo because he had earlier rediscovered another "lost" specimen—a stuffed Sumatran muntjac collected in 1914—in London's Natural History Museum.
Until the British biologist's museum discovery, the second muntjac species had been largely forgotten by science for some 60 years.
"This deer might well be relatively common in Sumatra above a certain altitude," said Timmins, speaking from his base in Madison, Wisconsin. "I always suspected it still survived in the region."
Now been confirmed as a distinct species, the Sumatran muntjac has been placed on the global Red List of Threatened Species. (See photos of 2008 Red list animals.)
—James Owen in London