Pelagonia (Пелагонија; Πελαγονία). A district in Macedonia. The Pelagones were an ancient people, probably of Pelasgic origin, and seem originally to have inhabited tha velly of the Axius, since Homer calls Pelagon, a son of Axius. The Pelagones afterwards migrated westwards to the Erigon, the country around which received the name of Pelagonia, which thus lay south of Paeonia. The chief town of this district was also called Pelagonia (now Vitolia or Monastir), which was under the Romans the capital of the fourth division of Macedonia. It was situatied on the Via Egnatia, not far from the narrow passes leading into Illyria.

Sources:

Smith, William. "A Classical Dictionary of Biography, Mythology, and Geography". London: John Murray, 1878. p. 535.