A horse’s neigh may be unique in the animal kingdom. Now scientists know how they do it

A few small rodents like rats and mice whistle like this, but horses are the first known large mammal to have a knack for it.
Nature

Whistle while you whinny: researchers identify two sounds straight from the horse's mouth

The animals’ ability to whistle while vocalizing earns them membership into a small club of creatures that can make biphonic sounds, which includes several birds species, human beat boxers, Mongolian ...