The Mongolian fermented mare’s-milk beverage is called airag.
Airag is made by fermenting raw mare’s milk, traditionally in a leather container and with repeated stirring or churning. Lactic-acid bacteria sour the milk, while yeasts produce carbon dioxide and a small amount of alcohol. The finished drink is tart, lightly fizzy, and mildly alcoholic rather than sweet like fresh mare’s milk.
The beverage is central to pastoral life in Mongolia, where horses provide both transport and dairy products. The traditional milking season generally runs from mid-June to early October. A mare may produce roughly 1,000 to 1,200 litres of milk during a season, though about half is left for her foal.
Kumis is the main source of potential confusion. It is a Turkic-derived name for the same broad fermented mare’s-milk tradition, while airag is the Mongolian name. Thus “kumis” is not a different Mongolian beverage in this context, but airag is the most precise answer to a question specifically asking for the Mongolian term. Historical accounts also describe the drink: William of Rubruck recorded Mongolian preparation in the 13th century, and earlier writers described related steppe traditions.