User blog comment:Selim 009/Seven Dragons Vs. Seven Dragon Slayers/@comment-7418318-20130428194139

There is a 7th Dragon Slayer - Cobra.

While he is "only" 2nd generation, he was always dominating the Fairy Tail members pretty well, untill something happened to turn the tide (one time being shot in the back and another time Kinana showing up).

He is currently sitting in prison, but maybe the favour Jellal asked Doranbolt and Lahar is to release Cobra, so he can fight against the Dragons.