The attitude of a soldier, mounted on a rampant horse and thrusting a lance, derives from classical models. There's a 5th C carving of Bellerophon (mounted on Pegasus), and a roughly contemporary ivory (the "Barberini diptych") of the Christian emperor Anastasius, to cite two secular examples. The earliest depictions of St. George are usually standing, not mounted, in which he's paired with another soldier-saint (Demetrios or Theodore). In a quick search of reproductions in my library, I'm not able to find a pre-Islamic icon of George on horseback; the earliest such icon I've found so far is from St. Catherine's in the Sinai, thought ot be of Georgian derivation, and shows the saint with his lance poised over a man, perhaps the emperor Diocletian, who touches the lance-tip in submission. Remember that virtually all soldier-saints are martyrs, and that all martyrs are crowned (representing victory, especially of the sort proper to a spiritual struggle). IMO the intent of the iconographer in depictions of soldier-saints in combat is to make visual reference to such struggles.
St. George
Martyr, patron of England, suffered at or near Lydda, also known as Diospolis, in Palestine, probably before the time of Constantine. According to the very careful investigation of the whole question recently instituted by Father Delehaye, the Bollandist, in the light of modern sources of information, the above statement sums up all that can safely be affirmed about St. George, despite his early cultus and pre-eminent renown both in East and West (see Delehaye, "Saints Militaires", 1909, pp.45-76).[...]
SAINT GEORGE AND THE DRAGON
The best known form of the legend of St. George and the Dragon is that made popular by the "Legenda Aurea", and translated into English by Caxton. According to this, a terrible dragon had ravaged all the country round a city of Libya, called Selena, making its lair in a marshy swamp. Its breath caused pestilence whenever it approached the town, so the people gave the monster two sheep every day to satisfy its hunger, but, when the sheep failed, a human victim was necessary and lots were drawn to determine the victim. On one occasion the lot fell to the king's little daughter. The king offered all his wealth to purchase a substitute, but the people had pledged themselves that no substitutes should be allowed, and so the maiden, dressed as a bride, was led to the marsh. There St. George chanced to ride by, and asked the maiden what she did, but she bade him leave her lest he also might perish. The good knight stayed, however, and, when the dragon appeared, St. George, making the sign of the cross, bravely attacked it and transfixed it with his lance. Then asking the maiden for her girdle (an incident in the story which may possibly have something to do with St. George's selection as patron of the Order of the Garter), he bound it round the neck of the monster, and thereupon the princess was able to lead it like a lamb. They then returned to the city, where St. George bade the people have no fear but only be baptized, after which he cut off the dragon's head and the townsfolk were all converted. The king would have given George half his kingdom, but the saint replied that he must ride on, bidding the king meanwhile take good care of God's churches, honour the clergy, and have pity on the poor. The earliest reference to any such episode in art is probably to be found in an old Roman tombstone at Conisborough in Yorkshire, considered to belong to the first half of the twelfth century. Here the princess is depicted as already in the dragon's clutches, while an abbot stands by and blesses the rescuer.