In Caxton’s work, Coldroe rode a horse with the tail of a peacock, the rump of a partridge, a tiny head, and a coat that was fiery red on one side and “white as a fleur de lis” on the other. I gather that medieval audiences loved that sort of thing, but it jangles against the modern taste. I thought about changing it to a hippogriff or griffin, but both of those can fly (which would ruin the scene) and magic items are familiar enough to slide seamlessly into our fictive dream. I can’t be sure, but it wouldn’t surprise me if the saddle would jar the medieval mind as much as the feathered horse would have done to us.
Please note that the white hart was a common literary symbol. I’m told that it represents magic at play, in this case the hand of Providence. A medieval audience would have immediately understood that the scene shows an actual miracle in progress.
It’s also worth noting the horns of Ganelon’s dilemma in both this Chapter and the next one. He’s caught between his family ties and his loyalty to the King.