Herodotus, who provides the only surviving testimony, informs us that river Angros (modern West Morava and its tributary Ibar) enters the Triballian plain and flows into river Brongos, while Brongos – into Danube (Hdt 4, 49). Two more variants Βάργος and Μάργος were mentioned by Strabo.

Tomaschek identifies the name as Thracian, unlike the other name, Μάργος, which he considers to be Celtic. According to Tomaschek Βρόγγος is comparable to the name Brunga, a road station in Bithynia, which is highly probable. D. Detschew includes the name in the Thracian linguistic remains, as well, and accepts the etymology suggested by Tomaschek. 

It is also possible that Βάργος and Βρόγγος are variants of the same name, but the lack of reliable data on other cases of such a relationship makes it impossible to draw firm conclusions. The etymologies remain uncertain.