Well I heard a theory about Dragon's past. Well it is about an ancient japanesse rumor that says: There was a king (World Governement) that had four underlings. A bird (Dragon), a monkey (Kizaru), a dog (Akainu)and a blue pheasant (Aokiji). Then the bird realized that the king was mean, but the monkey was a bit stupid and it didn't understand, the dog was very loyal to the king and the pheasant was too lazy to leave. In addition when Smoker saw Dragon in Logetown he didn't attack him despite the fact that he was a criminal. He might have done it because he respected him as we know that Smoker will attack any criminal no matter how strong he is.Also Garp is the strongest marine ever, so it wouldn't be a problem for Dragon to join. Although,despite this, Thalassa Lucas said eight years before the start of the story that Dragon was not well-known yet, so the theory now seems pretty dumb.