I might be mistaken, but I think that the Dukemon (Gallantmon)/Megidramon relation is actually a reference to the knight Siegfried and the evil dragon Fafnir. All legends say that Siegfried was a valorous and kind knight that used all his power to defeat Fafnir before he became too much of a threat and, after the battle, bathed himself in Fafnir's blood and, in doing so, took the dragon's power for himself. Thus he became both, a dragon slayer and a dragon knight. However, one version of the legend say that, along with the blood, Siegfried also took a piece of Fafnir's conciousness as well, wether it was on purpose or not. And this fragment of Fafnir is constantly seeking to either, take control of Siegfried, or corrupt him. The story then cautioned that, should Fafnir suceed in either of his objectives, then Siegfried would become a draconic creature far more powerful and evil than Fafnir himself could ever hope to be. Powerful enough to bring about the ruination of the entire world. From what Digimon Tamers and the wikia say, the is very much similar to what happened to Takato and Guilmon.