Known as Przewalski’s horses, the ancient species has narrowly avoided extinction thanks to breeding programmes at zoos worldwide and is now gradually being re-introduced to a wildlife reserve in its original homeland.

But confined to wooden boxes, Finnish sisters Helmi and Hanna, German mare Spes and Yanja from a Swiss zoo are not enjoying the 30-hour trip to get to their new home one bit.

“The plane trip is the toughest part,” Prague Zoo chief vet Roman Vodicka told AFP over the constant drone of the twin-engine propeller plane. Before being enclosed in the transport boxes, they were put to sleep, tested and treated by vets. Then they were taken by truck to a military airbase on Prague’s outskirts and loaded onto the plane.

Prague Zoo, which has bred Przewalski’s horses since 1932 and keeps the world genealogy book for the endangered species tracking all new births, launched a project to reintroduce the animals to Mongolia in 2011.

The four round-bellied, short-legged, sandy-beige mares are set to join wild herds in Takhin Tal (meaning the wild horse steppe), where 220 Przewalski’s horses now gallop free.

In 1969, there was just one.