The Dragon Emperor is more of a label that pure-bred dragons used for themselves. Since they were able to breed with any living thing, the number of subspecies they created was immense, but when they were being perused to extinction, they sought a to survive by combining their cores into one being, the Dragon Emperor. After the fight with Fitoria the core of the Dragon Emperor scattered into shards and their/his consciousness was distributed to the cores of the dragon subspecies. Gaelion's body (the one that turned into the Zombie Dragon) was a subspecies that happened to have enough Dragon Emperor fragments recognize himself as a portion of the Dragon Emperor and Kogaelion's consciousness is slowly merging with the Dragon Emperor.