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.
Oghuz Turks
During the 2nd century BC, according to ancient Chinese sources, a steppe tribal confederation known as the Xiongnu and their allies, the Wusun (probably an Indo-European people) defeated the neighboring Yuezhi and drove them out of western China and into Central Asia. Various scholarly theories link the Xiongnu to Turkic peoples and/or the Huns. The first usage of the word "Oghuz" appears to have been the title of "Oğuz Kağan", given in 220 BC to the Xiongnu king Modu Shanyu (or Mau-Tun), who founded the Xiongnu Empire. According to a controversial theory with few scholarly adherents, one transliteration of Yuezhi, as "Hu-chieh", may refer to the Turkic Uyghurs. However, the Yuezhi are widely believed to have spoken an Indo-European language or languages.