Rotom is not legendary.
while its true that it does not evolve
and that you only get one chance to get
That does not mean its legendary.
take Pokemon Collosseum for an example. you get ONE chance to steal a blaziken. it does not evolve and you got ONE chance. along with any other pokemon. a last example pokemon emerald you get ONE chance to get it, it doesn't evolve, and its got its own theme music and you can't breed it. so HAH!