Oncus or Oncius was an Arcadian king and a son of Apollo. He is best known for his horse Arion, which he presented to Heracles. We are not told a great deal about him, and hardly enough to explain why his name seems to mean 'bend' or 'hook' (oncos).

On the other hand, oncos could also mean 'bulk', 'weight', 'importance' (and conversely 'arrogance'), so perhaps he was either 'important' or 'conceited'.