While the late 20th century assumed all large land mammals had been catalogued, a massive unknown creature was hiding in the remote Annamite Mountains between Vietnam and Laos. Discovered in 1992, the saola, or Asian unicorn, rewrote textbooks as the first large mammal found in the region in 50 years, and it may already be slipping toward extinction.

This episode explores how an animal this size stayed hidden, what makes its biology so bizarre, and why scientists are now attempting a desperate, high-stakes rescue. Despite its delicate antelope-like appearance, DNA revealed the saola is the earliest diverging member of the cattle family, an evolutionary outlier on a genetic knife edge for millennia.

• How a strange skull in a hunter's kitchen, not a grand expedition, sparked the discovery
• The taxonomic twist that placed this ...