The mongols incorporated the into their military forces.
olmecs
egyptians
uyghur turks
vikings

The military forces that was incorporated by the Mongols are the Uyghur Turks. They are a group of small nomadic tribes from Altai Mountains. After the Uyghur Khagante collapsed, they resettled from Mongolia to Tarim Basin at which they were driven out of the region by the Xiongnu. then they eventually became the civil servants that administer the Mongolian empire.

Primary sources are usually  more accurate and are written by someone who experienced it or witnessed it. secondary sources are those who heard of it after the fact that can lose details and or twist the event in a light that benefits them.