The Khotanese language was the successor of older East Iranic shaka language. A major bauddha text in it has come down to us in fragments: the book of Zambasta. It mentions the invasion of Khotan by 5 enemies who harmed Khotan: mAMkuya, Red Khocas, huna-s, ciMgga-s, supIya-s.
mAMkuya rro Inda heinA kho—ca u huna ciMgga supIya | kye naa hvataana-kShIru bajo—ttanda ttu ju ye gSvu ne oysde ||
Emmerick translated this as:
There are mAMkuya-s, Red Khocas & hunas, Cimggas, Supiyas, who have harmed our Khotanese land. For a time one has not been angry about this.
Bailey in the 1950s itself recognized that what was rendered as mAMkuya, would be originally something like monguya in a Turkic or Mongolic language. Now cimgga-s are well known to be the chIna-s who ravaged Khotan & still claim it. supIya-s are have been identified as the Tibetans. The red Khoca-s have been identified as the Tuyuhun (using chIna orthography) khaghnate that emerged from the Xianbei as their faces have been found painted red in their grave sites. The hUna-s have been identified the Xiongnu khaghanate. So who were the monguya?
The invaders that conquered Khotan entirely that are not accounted for so far are the Rouran Khaghanate. Golden notes that one of their founding Khans was specified as Muguluu & the chIna transcription probably left out the nasal. Thus, maMkuya were probably the horde of Muguluu.
This would be the earliest record of the Mongol ethnonym & plainly connect them to the Rouran Khans. Vaissiere who read the Khüis Tolgoi brAhmI inscription as Mongolic thinks this is indeed the case & that it was made by the said khaghanate. It would match with the Jiu Tangshu compiled by the chIna-s in 945 CE but recording events in the heydays of the Tang where the term Mengwu is mentioned corresponding to something like Monggo.