Proto-Mongolic, the ancestor language of the modern Mongolic languages, is very close to Middle Mongol, the language spoken at the time of Genghis Khan and the Mongol Empire. Most features of modern Mongolic languages can thus be reconstructed from Middle Mongol. An exception would be the voice suffix like -caga- 'do together', which can be reconstructed from the modern languages but is not attested in Middle Mongol.
The languages of Donghu, Wuhuan and Xianbei might be related to Proto-Mongolic, as might be Tabghach (the language of the founders of the Northern Wei dynasty) and Khitan. In the case of Tabghach, the surviving evidence is very sparse, thus one can state that a generic relationship is possible. In the case of Khitan, there is rich evidence, but most of it is written in the two Khitan scripts that have as yet not been fully deciphered. However, from the available evidence it has to be concluded that a generic relationship to Mongolic is likely.
Read more about this topic: Mongolic Languages