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
Harold Walter Bailey proposed an Iranian origin of the Xiongnu, recognizing all the earliest Xiongnu names of the 2nd century BC as being of the Iranian type. This theory is supported by turkologist . Central Asian scholar Christopher I. Beckwith notes that the Xiongnu name could be a cognate of Scythian, Saka and Sogdia, corresponding to a name for Northern Iranians. According to Beckwith the Xiongnu could have contained a leading Iranian component when they started out, but more likely they had earlier been subjects of an Iranian people and learned from them the Iranian nomadic model.