What is "xueyantuo"

The Xueyantuo (薛延陀) (Seyanto, Se-yanto, Se-Yanto) or Syr-Tardush were an ancient Tiele Turkic people and Turkic khanate in central/northern Asia who were at one point vassals of the Gokturks, later aligning with China's Tang Dynasty against the Eastern Gokturks. Xueyantuo started from Selenga River/ Xueyanhe River (薛延河江/偰輦河江), so tribe name is Seyanto/Xueyantuo (薛延陀), Chinese Han character undergo considerable revision by rise and fall of Chinese Dynasty, so have many name as Xueyantuo, Xueyanhe, Xienianhe, Seyanto, Selenga, Selyanha, etc.