Saint Magnus appears to come into the story sometime between the 16th century and 1690. Down in Caithness a story was told of the saint turning a dragon into the Stone Hone in the Watten parish, now reduced to rubble.
The first mention is that a man was shipwrecked and got home by jumping on the back of a monster, which he then turned into the stone, this “Gallus” promising to dedicate a church to St. Mary. Although the writer wrote of the stone being by a ‘temple’ near the shore I presume this simply means the kirk. Between this time and 1701 it seems to have been reduced from 6’x4’ to the present dimensions – perhaps this was to remove some pagan feature.
By Gallus we might be dealing with another word for a RC priest – perhaps Magnus’ name was attached to protect the stone at a time of religious upheaval. Of course as this saint was an earl of virtually kinglet status it could have been attached even earlier.