Dragon was a former admiral, a theory based on many threads i have seen on forums. First of all there is an old Japanese myth about a King (WG) who has four loyal followers - first was the bird (Dragon), second was the Dog (Akainu), The Second Monkey (Kizaru) and the fourth one A Pheasant(Aokiji).

So what happens is that bird(Dragon) realized king was evil, so he decided to leave him but he was a good friend of Dog, Monkey and Pheasant so he also told the same to them. but Dog was too faithful, monkey was too dumb and Pheasant was too lazy to do anything.

So this explains how he is the most wanted man after leaving world Govt. he is declared as the most Wanted Criminal of the world.

Also Garp Wanted Luffy to be in Marine, so do you think he never wanted his Son Dragon to be in Marines. He always wanted his son to be in Marines and he joined them so but he left became the most wanted criminal in the world.

Also there is another myth about Dragons that "Dragons are mythical creatures in control of wind, weather and all elements."

This explained how he can control the weather and wind in Loguetown.

I have some kind of Proof in the image of old marines gathered in a picture.