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
Since the early 19th century, a number of Western scholars have proposed a connection between various language families or subfamilies and the language or languages of the Xiongnu. Albert Terrien de Lacouperie considered them to be multi-component groups. Many scholars believe the Xiongnu confederation was a mixture of different ethno-linguistic groups, and that their main language (as represented in the Chinese sources) and its relationships have not yet been satisfactorily determined. Kim rejects "old racial theories or even ethnic affiliations" in favour of the "historical reality of these extensive, multiethnic, polyglot steppe empires".