PARIS, Nov 3 (Reuter) - Explorers have discovered a previously unknown breed of small and primitive forest pony in a remote part of Tibet, the expedition's leader said on Friday.

Frenchman Michel Peissel, who led the seven-week expedition to the Riwoche area in north-eastern Tibet, told Reuters the pony "resembles the horses depicted in neolithic caves."

"It has a rectangular face, a bristly mane and stands about 120 cm (four feet) high at the shoulder," he said, adding that local inhabitants use the animal as a work horse.

"This really is a breakthrough," Peissel said. "We were not expecting to stumble on this so we were unable to bring one out. But when we have the necessary permissions we plan to return."

Peissel, who has been exploring the Himalayas for 36 years and has written 15 books about his travels, led a Franco-British expedition to Tibet in 1994 that discovered the source of the Mekong River.

The year before, he led another expedition which discovered a larger purebred horse also unknown outside Tibet. He has dubbed it the Nangchen horse because it came from Tibet's Nangchen region.

The animal's enlarged lungs and heart enable it to survive in the mountainous area's rarified atmosphere.

The six-man expedition just completed was planned primarily to bring a Nangchen horse out of Tibet for detailed study -- a task his team was unable to carry out due to heavy snows, he said. But films of the expedition have been made and will eventually be made public, he said.

Two years of negotiations were required to obtain permission for the visit from the Chinese authorities, who have closed off large parts of Tibet to foreigners. Peissel, who returned to Paris from Tibet late on Thursday, said the remote regions visited by his team were among "the very few unexplored areas left in the world."

"To our surprise, we found immense forests," he said.

Snow-bound passes, hailstorms and steep drops along the trails followed by the explorers on horseback made the journey one of the most exhausting and dangerous of his 24 expeditions to the Himalayas region, he said.

Peissel, who speaks fluent Tibetan, is a member emeritus of the New York Explorers Club and a Fellow of Britain's Royal Geographical Society. His expeditions have been featured in the National Geographic magazine and on the BBC.