Final confirmation of the new species has to await the genetic analysis of material taken from the bones of the deer but the biologist heading the investigation, John MacKinnon of the World Wide Fund for Nature, is confident the animals will turn out to be novel.
'In terms of mammal discoveries, this is the biggest news this century,' Dr MacKinnon said. The two new species come hot on the heels of the discovery of the Vu Quang ox, an ancient relative of modern cattle named after the nature reserve in Vietnam where it was found, and the giant muntjac, a deer- like animal with a passing resemblance to Bambi.
One of the possible new species is known locally as the slow-moving or slow-running deer and is 'very strange indeed', Dr MacKinnon said. The only evidence he has of its existence are six skulls collected from local hunters. The animals have two short, sharp antlers sticking out like the horns of a Viking helmet.
The second possible new species is an animal known locally as the 'mangden', or black deer. Dr MacKinnon has only a single set of antlers of the species but said they appear to be distinct from the antlers of other deer living in the area.
Until the discovery of the Vu Quang ox - Pseudoryx nghetinhensis - in May 1992 no new large mammal had been found for 50 years. Zoologists were astounded that an animal the size of a large dog could have escaped their attention. They are even more amazed at the prospect of three new species of deer.
Dr MacKinnon and his colleagues discovered the giant muntjac last March after analysing skulls and other trophies kept by local hunters. Dr MacKinnon said the deer, which appears to be quite common in the area, has been seen but no one has yet managed to capture a living specimen.
Dr MacKinnon said the finds are a classic example of 'endemism' in biology - the evolution of distinct species within sharply defined geographical areas, usually rugged mountains, where animals can be isolated for hundreds of thousands of years.
The Vu Quang nature reserve and the Pumat mountain jungle, situated about 50 miles further north on the Laos-Vietnam border, are prime examples of pristine 'lost worlds' where endemism has thrived. As well as steep slopes, which make the area out-of-bounds for many four-legged animals, the region is marked by its almost continuous rains and mists, which encourage the growth of a slippery algae on the ground, creating an even more treacherous toe-hold.
''It can rain for 10 days without a five-minute break - and that's in the dry season,' Dr MacKinnon said. 'It seems to be a refuge for more archaic animals that have not been overwhelmed by modern species. It's very wet and slippery and almost impossible to walk there. These animals seem to cope well.'
Added to the inhospitable climate, these heights have been forbidden territory to western scientist and explorers because of the communist regimes of Laos and Vietnam and years of hostility during and after the Vietnam war.
So far only one specimen of the Vu Quang ox - a young female - has been captured alive and the fear is that these new species could be driven to extinction very quickly if the surrounding forests are opened up for commercial logging. The living specimen, which is thin but apparently healthy, is living in Hanoi having been confiscated from local hunters by the Vietnamese government.
Dr MacKinnon believes the discovery of large mammals in the area is only the 'spectacular tip of the iceberg'. He said there is a strong possibility of finding many new species of smaller animals - for instance bats and shrews - once scientists make a concerted effort to collect them.
One of the lessons of Vu Quang, he said, is that remote mountain areas in other parts of the world could still be hiding new species of large animals. This week Dr MacKinnon is in the Himalayas - the alleged home of the elusive Yeti. Does he think something the size of a Yeti could exist? 'I keep an open mind on that one,' he said.