SOSIPOLIS was a child god, a son of Eileithyia, goddess of childbirth. He was worshipped by the Eleans as the protector of their city and depicted as a boy clothed in a star-spangled robe with a cornucopia or Horn of Plenty.
Sosipolis may have been identified with Ploutos, who was also represented as a child holding the horn of plenty, or else with Eros, who in some old hymns was described as the son of Eileithyia.
SOSI′POLIS (Sôsipolis), i. e. the saviour of the state, was the name of a hero among the Eleans, who was represented as a boy wearing a military cloak, and carrying the horn of Amalthea in his hand. He had a sanctuary in common with Eileithyia at the foot of the hill of Cronos at Olympia, and no one was allowed to approach his altar except the priestess, and even she only with her head covered. Oaths in which he was called upon were considered to be particularly solemn and binding. The origin of his worship is thus related :-- Once when the Arcadians had invaded Elis and the Eleans had marched out to meet them, there appeared among the Eleans a woman with a boy at her breast and declaring that after she had given birth to the child she had been called upon by a vision in a dream, to offer the child as a champion to the Eleans. The commanders of the Eleans believing the assertion, placed the child naked before their ranks, and when the Arcadians began the attack, the child was metamorphosed into a serpent. Hereupon the Arcadians fled in dismay, and the Eleans pursuing them gained the victory. The Eleans hence called their saviour Sosipolis, and on the spot where he had disappeared in the form of a snake they built a sanctuary to him and his supposed mother Eileithyia. (Paus. vi. 20. § 2, iii. 25. §4.)
Source: Dictionary of Greek and Roman Biography and Mythology.
Pausanias, Description of Greece 6. 20. 2 (trans. Jones) (Greek travelogue C2nd A.D.) :
"At the foot of Mount Kronios (Cronius) [at Olympia, Elis], on the north, between the treasuries and the mountain, is a sanctuary of Eileithyia, and in it Sosipolis (Saviour of the State), a native Elean deity, is worshipped. Now they surname Eileithyia Olympia, and choose a priestess for the goddess every year. The old woman who tends Sosipolis herself too by an Elean custom lives in chastity, brining water for the god’s bath and setting before him barley cakes kneaded with honey. In the front part of the temple, for it is built in two parts, is an altar of Eileithyia and an entrance for the public; in the inner part Sosipolis is worshipped, and no one may enter it except the woman who tends the god, and she must wrap her head and face in a white veil. Maidens and matrons wait in the sanctuary of Eileithyia chanting a hymn; they burn all manner of incense to the god, but it is not the custom to pour libations of wine. An oath is taken by Sosipolis on the most important occasions. The story is that when the Arkadians (Arcadians) had invaded the land of Elis, and the Eleans were set in array against them, a woman came to the Elean generals, holding a baby to her breast, who said that she was the mother of the child but that she gave him, because of dreams, to fight for the Eleans. The Elean officers believed that the woman was to be trusted, and placed the child before the army naked. When the Arkadians came on, the child turned at once into a Drakon. Thrown into disorder at the sight, the Arkadians turned and fled, and were attacked by the Eleans, who won a very famous victory, and so call the god Sosipolis. On the spot where after the battle the snake seemed to them to go into the ground they made the sanctuary. With him the Eleans resolved to worship Eileithyia also, because this goddess to help them brought her son forth unto men."
Pausanias, Description of Greece 6. 25. 4 :
"Here [in Elis] Sosipolis too is worshipped in a small shrine on the left of the sanctuary of Tykhe (Tyche, Fortune). The god is painted according to his appearance in a dream: in age a boy, wrapped in a star-spangled robe, and in one hand holding the horn of Amaltheia."
- Pausanias, Description of Greece - Greek Travelogue C2nd A.D.