» those thousand words ♥
A much later point, that is. Keep in mind that just about almost every single legendary that has ever had an alternate form had it's form announced pretty late in the game. Take Giratina for instance. It existed in D/P sure, but didn't have it's alternate form announced until Platinum. Gen V also went towards this route, Kyurem not having an alternate form until B2W2 were just about around the corner. The Kami trio were the same way also.
But it's also important to note that these Pokemon were also the "third" legendary Pokemon (except Kami trio), hence it seems like they're usually the ones to get alternate forms, rather than the initial main mascots.