Four rare Przewalski's wild horses were headed for the Mongolian steppe from Prague on Monday as part of a project to reintroduce the critically endangered species to its ancient homeland.

Prague zoo runs a breeding programme and is charged with keeping the world genealogy book for the equines which have survived only in captivity since the last wild horse was seen in Mongolia in 1969.

The four mares, aged three to five, will be flown 18 hours in wooden boxes to Bulgan, about 200 kilometres (125 miles) northwest of the Mongolian capital Ulan Bator.

"The mares are more nervous than last year," Prague zoo director Miroslav Bobek said, referring to another four horses sent to Mongolia in 2011.

From Bulgan, the horses will be taken by road to the Gobi B national protected park, where the population of the horses shrank to 49 from 150 during the severe winter of 2009-2010.

"They must be animals of an adequate age, with the correct genetic setup, in a good shape, and we also chose them by character to avoid taking mares susceptible to stress," Bobek said.

Two mares sent last year have already given birth in the steppe.

"We'd like to continue with the transports to Mongolia... where we're trying to create sustainable conditions for both the horses and the people who live close to them," said Bobek.

Characterised by thick necks, large heads and stocky girths, Przewalski's horses weigh between 250-350 kilograms (550-770 pounds) and stand about 1.2-1.3 metres (3.9-4.3 feet) tall at the withers.

With ancient cave paintings in Lascaux, France, featuring wild horses eerily resembling the Przewalski variety, the species is thought to have lived in Europe 20,000 years ago, but climate change chased the animals away to Asia.

Europeans only encountered the wild horses at the end of the 19th century when Russian explorer and geographer Nikolai Mikhailovich Przewalski discovered them in mountains bordering the Gobi desert.

In the 20th century, hunting brought Przewalski's horses to the verge of extinction.