The Mantu are an alien race with territory somewhere beyond that of the Kilrathi, opposite the Terran Confederation. The Mantu engaged in armed conflict with the Kilrathi approximately two centuries before Human/Kilrathi contact. This conflict ended with a stalemate, and it is believed that the Mantu may currently be involved in a conflict on their far border, away from Kilrathi space.
The Kilrathi have elevated the Mantu to a nearly mythic level. Before their encounter with humanity, the Mantu were the only species able to hold their own for any period of time against the Kilrathi. This myth loomed over the Kilrathi throughout the Terran-Kilrathi War. For instance, one of the motivating factors behind the construction of the Hakaga carriers was to quickly end the war with the Confederation so that the Mantu could not attack while the Empire was already engaged. After the Confederation was defeated, the Hakaga class ships were to be used to defend the Empire against the Mantu attack that Thrakhath believed would occur as soon as they dispatched with their distant enemies.
Source: WC CIC Encyclopedia