AGON was the spirit (daimon) of contest, who possessed at altar at Olympia, site of the Olympic Games.
He may be the same as Zelos (Rivalry), brother of Nike (Victory).
AGON (Agôn), a personification of solemn contests (agônes). He was represented in a statue at Olympia with haltêres in his hands. This statue was a work of Dionysius, and dedicated by Smicythus of Rhegium. (Paus. v. 26. § 3.)
Source: Dictionary of Greek and Roman Biography and Mythology.
Pausanias, Description of Greece 5. 20. 3 (trans. Jones) (Greek travelogue C2nd A.D.) :
"[Depicted on table made by Kolotes (Colotes) and dedicated at Olympia:] Behind is the disposition of the games. On one side are Asklepios and Hygeia (Health), one of his daughters; Ares too and Agon (Contest) by his side."
Pausanias, Description of Greece 5. 26. 3 :
"Among the offerings of Mikythos (Micythus) [at Olympia] is [a statue of] Agon (Contest) carrying jumping-weights."
- Pausanias, Description of Greece - Greek Travelogue C2nd A.D.