In Greek mythology, Oncius[pronunciation?] (Ancient Greek: Ὄγκιος, romanized:Ónkios, lit.'case for arrowheads') or Oncus (Ancient Greek: Ὄγκος, romanized:Ónkos, lit.'point of an arrow') was a son of Apollo and a ruler over Ónkeion (Ὄγκειον), a region of Arcadia adjacent to Thelpusa, as well as eponym of a city Oncae. He owned a herd of horses, in which Demeter tried to hide from Poseidon's advances, changing herself into a mare. Poseidon did mate with her in the shape of a stallion, which resulted in the birth of the fantastic horse Arion. Oncius kept Arion and later gave him away to Heracles as the latter was starting a military campaign against Elis.
Stephanus of Byzantium, Stephani Byzantii Ethnicorum quae supersunt, edited by August Meineike (1790-1870), published 1849. A few entries from this important ancient handbook of place names have been translated by Brady Kiesling. Online version at the Topos Text Project.