History of Mongolia
Although the Xiongnu finally had been split into two parts in AD 48, the Xianbei (or Hsien-pei in Wade–Giles) had moved (apparently from the east) into the region vacated by the Xiongnu. The Xianbei were the northern branch of the Donghu (or Tung Hu, the Eastern Hu), a proto-Mongol group mentioned in Chinese histories as existing as early as the 4th century BC. The language of the Donghu is believed to be proto-Mongolic to modern scholars. The Donghu were among the first peoples conquered by the Xiongnu. Once the Xiongnu state weakened, however, the Donghu rebelled. By the 1st century AD, two major subdivisions of the Donghu had developed: the proto-Mongolic Xianbei in the north and the Wuhuan in the south.
Xiongnu
An early reference to the Xiongnu was by the Han dynasty historian Sima Qian who wrote about the Xiongnu in the "Records of the Grand Historian" ( BC), drawing a distinct line between the settled Huaxia people (Chinese) to the pastoral nomads (Xiongnu), characterizing it as two polar groups in the sense of a civilization versus an uncivilized society: the Hua–Yi distinction. Pre-Han sources often classify the Xiongnu as a "Hu" people, which was a blanket term for nomadic people in general; it only became an ethnonym for the Xiongnu during the Han.