What is "didouyu"

The Didouyu were a 5th-century west Manchuria located steppe tribe, their neighbourhoods were the Tujue (Eastern Turk, on their west), the Rouran, and the Khitan (on their southeast). They were attacked by both Goguryeo and Rouran in 479, allied for this operation. In fear, a big part of Kithan fled southward to submit to Northern Wei.