Kumis (called airag by the Mongolians), is a traditional drink of the people of Central Asia.
It is made from fermenting mare's milk in a horse-hide container, called a "saba". It may be mildly alcoholic, but one has to drink an enormous amount for it to have any effect.
The beverage was created first by the Tatars in the 13th century. It was originally made with the milk of the Tatar's horses and yeasts. When using a cow's milk, sugar should be added to ensure fermentation will carry on.
Generally kumis is about 2% alcohol (4 proof), but can be made more potent through distillation.
Kumis and its cousin Kefir are still produced in many parts of the world, and can easily be made at home.