The Mantu are a species who rule an empire far beyond the territories of the Kilrathi Empire.
During the 25th century, the Kilrathi and the Mantu were at war with each other. However, the war ended in a draw. Since then, the Mantu have virtually been forgotten by both humans and Kilrathi alike. Until the Terran Confederation won the Kilrathi War, the Mantu were the only race to survive a major conflict against the Kilrathi Empire.
It was partially because of the memories of the strong resistance of the Mantu that the Hakaga-class Carriers were built before the Battle of Earth. The carriers would have destroyed the Terran Confederation quickly and then pulled back into the Kilrathi Empire so that the Mantu could be finished off before they could take advantage of the Empire's temporary vulnerability. However, the humans destroyed most of the Hakaga-class Carriers when they won the Battle of Earth in 2668, forbidding the planned strike on both races.
While the Mantu are almost mythical nowadays, it is believed that they still maintain control over their portion of the galaxy. However, it is possible that they are once again at war with another race.