I don't want to be a downer, but I don't believe in the Isshu Dex and National Dex Rumors.
Let's try to guess:
After Zekrom, number 150 in the Isshu Dex, it has to exist a third Dragon/???
So let's assume Zekrom is the number #635.
The Third Dragon/??? is #636
#637 - Ice / Fighting...