Other things to consider: who said Acnologia is the only DS to become a dragon? Igneel, and metalicono all could have been human once. I'm positive Grandine was human once though.
The Jade Dragon made it sound like it was a rather unique event, i dont think there is anyone else that has transformed like him.

And furthermore, considering the bloodcrazed killingspree Arcnologia had to embark upon to transform himself, then i find it even more unlikely that any of the dragons we know of would have done the same.