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.

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.