User blog comment:IamJakuhoRaikoben/Chapter 431 & 432: My sword is... & Brair in Love, Review./@comment-7418318-20150429195415/@comment-9595671-20150430002339

When Natsu used Dragon Force the third time it was stated to be his true DF form. I assume when a source of power caused them to use Dragon Force that power can slightly alter its appearance.

Also DF seems to have only been able to be used willingly for the First Gen Slayers ever since the Dragons exited their bodies, like the reason they couldn't do so was because their very own Dragons were suppressing its power. I do recall the implying that the DF is what causes the transformation into a dragon, and when we saw Acnologia's human form he had horns which implies he had DF active.

In fact I think that makes Natsu's comment about Sting and Rogue's Dragon Forces being half-baked true at the time of the GMG because while they could use the Dragon Force, Weisslogia and Skiadrum's presence inside of them actively tried to suppress its power which weakened it. Had they fight without the Dragons inside of them, Natsu may have had more trouble.