While this isn't entirely a headcanon of mine, it's the plot of a fanfiction I'm writing and could make an interesting headcanon for some.
Ales Mansay was no good at magic. He failed magic school and got several tutors, but they all gave up on him, in one final attempt, Ales travelled to the Valley to meet with a limbless referring to himself as 'the Magician'. The introduction went well, and the Magician tried to teach Ales magic. Ales was no more successful with the Magician as a tutor, but he found himself deeply interested in the Magician's stories, particularly those about Mr Dark. As time went on, the two became rather good friends. However, Ales found himself admiring the cunning 'Mr Dark' more and more, and the irresponsible Polokus less and less. Eventually, the Magician caught on and scolded Ales for his backwards sense of admiration, but soon forgave Ales. who claimed that this wasn't the case at all.
Ales knew then that the Magician still trusted him, but any wrong move now could have that trust killed and the word could spread that Ales was in fact an admirer of an infamous villain. In order to prevent this, Ales took advantage of the Magician's trust by poisoning his tea and making him fall very ill. To make matters worse, when Rayman heard the news that the Magician was gravely ill, he didn't get back to the Valley in time to say his goodbyes. So while the emotion-fest was going on, Ales stole some books and other things from the Magician's house and headed back to the mainland of the Glade of Dreams to begin his plot to overthrow Polokus. Part of this involved once again taking advantage of trust. Ales knew that as soon as he made an obvious move they would have Rayman sent after him and that would surely ruin things, so he began imitating the appearance and mannerisms of Rayman's old mentor, the Magician. That way, not only would Rayman not be against him, but would be on his side and unknowingly helping him conquer the Glade.
Of course, then the events of Rayman Origins happened. He didn't show it much, but Rayman was pretty hurt and pissed off, with every right to be. A close friend and mentor of his had died after many years apart and no chance to say goodbye, then some random guy started acting and dressing like him, so he went along with it until the imitator betrayed them and went against everything the Magician ever stood for. If Rayman had known Ales killed the Magician he would have personally dragged the teensie through the depths of the Land of the Livid Dead.