Exactly what I said before...

Look, there is no evidence to support Reptile becomming Onaga (which would be The Dragon King's real name, so The Dragon King calling hiself Onaga doesn't prove he isn't Reptile) is canon or non-canon at this point. We don't even know if the way we understood the ending is right.

There are at least 101 ways Shujinko might feel guilty for the Dragon King's return. And at least 101 ways to explain this. I don't think it is really that relavent.

All we really know is that there is a prophesy. It could also be 101 different things. Who knows? My first instinct was that Reptile had somehow become The Last Dragon, so now Onaga has returned in an attempt to drink his blood, thus becomming immortal. So maybe he's hiding for the good of the realm's? He might be Super Unlockable. Who knows?