From what I have been able to gather by viewing some other people's scrolls and from some rumors I've seen on the Dragon Cave forum: there are 2 dragons that aren't regular "get an egg from the cave and grow it up" types. You have to do special things to get them. Unfortunately there isn't any good information on them because TJ has a strict rule about not posting "spoilers". Also, curiously, one of those 2 types has disappeared from the scroll I saw it on. So who knows what that means...
If people want, I can post what little more that I know here. Don't want to post "spoilers" though unless it seems that people want it.
Then maybe we can all work together to try to figure it out!