Board Thread:General Discussion/@comment-14410262-20131021231815/@comment-10090661-20131022000324

They are Gen-EX Dragons because they cannot be bred. Not the other way around.

If you look around, you will see that there are no "special" elements that "cause" a Dragon to be unbreedable.

Some Dragons are unbreedable because they are prizes -- it wouldn't make sense to offer a breedable Dragon as a main prize.