It's the army of Goguryeo. King Geun Gu-soo (or said to be whistle) is the son of the king of Geun-go. Prior to this, when King Goguryeo's King Seoyu invaded himself, the king of Geocho sent the prince to defend him. He came to Bangeolyang and tried to start the battle. The scam of Goguryeo was originally Baekjein, but he accidentally wounded the hoof of the horse on the king's ride. He fled to Goguryeo, fearing that he would be punished for this. He came back and said to the prince. “Although the number of Goguryeo soldiers are large, they are all just filled as fake soldiers. The strongest of them is the one with a red flag. If you attack the unit first, even if you don't hit the rest, it will be demolish on your own." -Three Kingdoms Sagi Volume 24, Baekje Bongi 2nd Third Country史記 卷第 Twenty-four 百濟本紀 第2, Korean History Database
Own work, I used Goguryeo Tomb Mural - Anak Tomb no.3 (357 A.D.) as a base for this flag.
Samhanin