Magus (Appearance of)
How did Janus's appearance change to that of Magus over his life in the middle ages?
The first possibility is that Magus somehow made his ears pointy artificially so as to differentiate himself from humans and gain favor with the Mystics. Janus was clearly a fairly intelligent child, and he was also the kind of person who believes he should get what he wants and is spoiled. However, despite all that, he couldn't have been more than 10 years old when the Ocean Palace disaster sent him to 600 AD, and if you suddenly tear a ten-year-old's life away from him, he's going to become very emotionally disturbed. Now, add this to the fact that he was extremely intelligent and powerful magically as a child as well as being spoiled, and you have a disturbed kid who wants revenge and knows how to get it. By the time he had appraised the 600 AD situation, he realized that the Mystics could not only keep him safe but also give him the power and resources he needed to extract revenge on Lavos. Of course, there remains the obvious problem that he is human. Now, if you were an intelligent person with powerful and unchecked dark magic that could be used for disguise at your disposal who found yourself in a world full of people who hate your race but could help you if only you were a different race, what would you do? Since humans were mortal enemies of the Mystics, he probably could not have rose to his station of power with his old appearance, instead opting to artificially alter his look to make it more conducive to his new friends. However, keeping up the image of having pale white skin would be troublesome, as makeup would have to be applied constantly. This leads to another alternative -- that Magus's new body was a result of overexposure to dark magic.
Dark Magic Exposure
Considering that Magus was raised by Ozzie for years in the dark confines of his lair practicing in the most arcane and darkest arts known, it is not out of the question that Magus's appearance accordingly changed with the experience. The effects might have also been forced upon him by Ozzie, who may have groomed the boy to his liking. In addition to the blue hair, pointy ears, and pale skin, Janus's irises have gone from green as a child to red as grown Magus. This suggests something a little more drastic than some artificial makeover to fit in with the Mystics -- unless they had colored contact lenses, which is doubtful. It is similarly unlikely that he could find a way to elongate his ears so well without the existence of plastic surgery.
From: Theory (People)