Yea I really thought they would delve more deeply into this concept. As of yet we have no idea what this mysterious Pokemon was like. If I remember correctly, wasn't Kyurem the empty shell that was left behind after the split? So based on that, I'd imagine the original one looked more similar to Kyurem, but without the "icy" features.
Also,I'm not sure what type it would have been. My best guess is a grass type, since at that time it was more in harmony with nature, as opposed to destroying it the way did when it split. Or it could have just been a pure Dragon type, and the elements didn't come along until after the split.
♥Paired with Dragon and Koakuma