Ancient Greek soothsayer who vanquished the Seer Calchas in a contest of their powers (FW).
According to the Prisoner of Azkaban video game, a painting of Mopsus resides in the dungeons at Hogwarts protecting a hidden room. The password to enter is "Forsooth" ( source)
Because of the accuracy of predictions made by Mopsus, the ancient Greeks had a saying "more certain than Mopsus." He was also one of the seers who aided Jason and the Argonauts (source: Wikipedia).
The date given -- 1300 B.C. -- is merely conjecture, of course. This is the date of the earliest recorded telling of the tale of Jason and his voyage to recover the Golden Fleece, a story in which Mopsus figures in a small way.