In May 1992, a team of field biologists set out to survey a remote patch of wilderness along the western border of Vietnam. After nine days on the trail, the group was running low on food, so two members were dispatched to a nearby village to stock up on provisions.

The two men were merely hoping to buy vegetables, but on the wall of a hunter’s shack, they stumbled across something spectacular: a pair of long horns, sleek, sharp and straight. The biologists had never seen anything like them.

The horns belonged to a saola, a species of wild ox previously unknown to science. “Suddenly, the scientific world had before it proof of a large, new living creature, previously unimagined,” writes William deBuys in his lyrical new book, “The Last Unicorn: A Search for One of Earth’s Rarest Creatures.” (Read excerpt.)

It was an animal unlike any other on Earth — not just a new species but a new genus, a mammal with no known close relatives. The saola has strange scent glands on its white-flecked muzzle and a preternaturally calm disposition. Viewed from the side, its two horns appear as one.