The Mongolians appeared in the Season Six episode, "Child Abduction is Not Funny", and in South Park: The Stick of Truth.
The Mongolians persistently broke down City Wok owner Tuong Lu Kim's city wall. Lu Kim was requested to build the wall to keep intruders from abducting South Park's children. Eventually, the children are sent away and join the Mongolians, due to the parents increasing paranoia and over protectiveness.
They all wear 13th-century-style clothing, have squinting eyes, and have long mustaches and beards. They carry swords as well as wooden shields.
The Mongolians appeared to be intelligent, easily outsmarting Lu Kim several times. Despite the fact that they destroyed the wall several times, they never entered South Park. The only reason given for their attacking the wall is that Mongolians try to break down any wall built by Chinese people.