The article analyzes the historical processes of the Middle Ages in the Baikal region, which led to the formation of an ethno-cultural oecumene, named Bargudzhin-Tokum. It is a well-known in historical literature and folklore semi-legendary country in the north of the Mongolian metaethnic community, a future ethnic Buryatia. The folklore ethnic genealogy of the Buryat people goes back to Bargudzhin-Tokum. The eponym "Bargudai-Mergen", the name of the leader of the Bargu tribe, was erected as the head. The three descending branches of genealogy are marked by the epochs Buriadai, Khoridoi and Oledoi that personify the Bargu, Khori and Oled (Oirad) tribal components. Historical and chronicle data for substantiating the Buryat genealogy are found in ancient Turkic monuments of runic writing, Rashid ad-Din's "Collection of annals" and Mongolian historical works of the 13^th–18^th centuries. The following sources were used to determine the ethnic substratum of the future Bargudzhin-Tokum population: the text of the epitaph of Kul-tegin (Second Turkic Kaganate, VIII century), where the story of the Baiyrku tribe is told the ancient Chinese historical source "Tan Shu", which refers to this same tribe under the name of Baiegu Mongolian historical works, which tells of the Bargu tribe. In the "Secret Legend of the Mongols," the historical act of the marriage of the royal dynasty of the Mongols with the Khori-Tumat tribe through a woman from Bargudzhin-Tokum was imprinted. Such marriages continued afterwards and became a tradition. According to the custom of the Mongolian peoples, the parents and all the relatives of a married woman are designated by the term “тɵрхɵм (Mongol.) and “түрхэм» (Buryat.), whose value, due to the great piety of the Mongols, was spread to the "Little Motherland" of the royal family. That is depicted, though in a slightly distorted form in the second part of the name Bargudzhin-Tokum. Therefore, we adhere to the etymology of this name, which means "Land of Bargu relatives" or "Congenerous Bargudzhin". For the first time in the practice of studying Buryat genealogy, such attention was paid to systematizing of information about Oirat ethnicities and searching for new data to clarify the previously obscure third branch of the Buryat genealogy. In the records of the Buryat folklore, general Mongolian and Buryat annalistic writings, it was possible to find quite reliable material that allows us to give a new version of the explanation of the Oled branch, which can be defined as the Tulass-Segenut.

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