You can't class Rotom as a legendary just because of the battle music played when you fight it. This music doesn't play when we battle Cresselia, and we still class her as a legendary. Plus, Rotom can breed, which also shows that it isn't a legendary. You can't use the idea of Manaphy to counter this, becuase Phione doesn't evolve back into Manaphy. Rotom also has less than average stats, so it shouldn't be classed as a legendary Pokemon.