Will we see Spiritomb's form before it was sealed? Surely they went and looked at Sinnoh mons and picked out which ones made sense to give a new ancient form. I don't think anyone could see Spiritomb and think it wouldn't be literal perfection to give it a new form in this game.

Yea 500 years ago isn't exactly when Legends takes place, but it's close enough to fudge it.