Nomadic empire
The Xiongnu were a confederation of nomadic tribes from Central Asia with a ruling class of unknown origin and other subjugated tribes. They lived on the Mongolian Plateau between the 3rd century BC and the 460s AD, their territories including modern day Mongolia, southern Siberia, western Manchuria, and the modern Chinese provinces of Inner Mongolia, Gansu, and Xinjiang. The Xiongnu was the first unified empire of nomadic peoples. Relations between early Chinese dynasties and the Xiongnu were complicated and included military conflict, exchanges of tribute and trade, and marriage treaties. When Emperor Qin Shihuang drove them away from the south of the Yellow River, he built the famous Great Wall to prevent the Xiongnu from coming back.
Proto-Mongols
The Donghu (or Tung Hu, the Eastern Hu), a proto-Mongol and/or Tunguz group mentioned in Chinese histories as existing as early as the 4th century BC. The language of the Donghu, unlike that of the Xiongnu, is believed by modern scholars to be proto-Mongolic. The Donghu were among the first peoples conquered by the Xiongnu. By the 1st century AD, the Donghu had split, along geographical lines in two: the proto-Mongolic Xianbei (Wade–Giles "Hsien-pei") in the north and the Wuhuan in the south. After the Xiongnu were driven back into their homeland by the Chinese (48 AD), the Xianbei (in particular) began moving (from apparently the north or northwest) into the region vacated by the Xiongnu.