Mongols

Mongolia is a mentioned only location.

Pre-War
is mentioned in the Pioneer Scouts archaeologist exam, specifically in the fact that his tomb was never discovered. The Trans-Siberian Railroad, part of which passes through Mongolia, is also mentioned in the exam.

Post-War
The Khans and New Khans, raider groups originating from Vault 15 in New California in the post-War United States, base their culture on the history of the Mongol Empire and that of Genghis Khan.

The remnants of these groups rebranded as the Great Khans and settled in the Mojave Wasteland. A book about the history of the Mongol Empire was found and brought to the region, entitled Pretty Pretty Horsies: A History of the Mongol Empire.

Appearances
Mongolia is mentioned in Fallout, Fallout 2, Fallout: New Vegas, and Fallout Shelter.

Behind the scenes

 * R. Scott Campbell described the Khans' inception, based on a real-life biker gang named after the Mongols.
 * Mongolia would have appeared in the canceled Fallout Extreme, home to a group of raiders known as the Huns.
 * The Mongols would have been mentioned in the intro to Fallout Pen and Paper d20.