Theory: Monkey D. Dragon is Uranus and was on Roger’s crew. Dragon could have possibly been born from the egg on Roger’s ship maybe fully grown because of the size of the egg not as a child. Or he was born as a dragon and takes the look of a man aka his usual look.
The egg on Roger’s ship is a dragon. Mostly seen in the Chapter 958 on the third page.
Chapter 958: https://jaiminisbox.com/reader/read/one-piece-2/en/0/958/page/3
Reasoning: Dragons in both Chinese and Japanese mythology have weather controlling powers and shape shifting powers, so Roger may have had control/allied with a dragon. During his battle with Shiki, Roger was only able to truly beat Shiki because of weird weather and a tidal wave destroyed most of his ships. Also, in that battle Shiki states that Roger has acquired information and/or the ancient weapon, Uranus. If that egg were to produce a dragon, then Roger would have power to control the weather thus the unusual storm + the tidal wave destroying Shikis fleet.
Moreover, right after Rogers death, the weather becomes very cloudy and rainy after being very sunny. If there were to be someone on Rogers crew who was mourning his death, the weather would typically be rainy to represent sadness right, so after his death, they’d be really sad and/or mad. In the clip of Rogers death, we can see all the pirates there reacting to his death. In the clip, we see Dragon walking way with an angry look on his face.
When do we also see Dragon in the rain in Loguetown, when he saves Luffy. When Luffy was about to be executed, a storm kicks in and a lightning bolt strikes the platform, but what if that isn’t just “Luffy’s luck”? What if it was Dragon saving his son knowing Luffy has the Gomu Gomu no mi from his connections through the Revolutionary Army. He then saves Luffy from Smoker, so he can continue his journey.
The reason why Dragon is so powerful and is so wanted is because he actually is Uranus, and the military/WG doesn’t have any control over an ancient weapon. He actually wants to take down the WG maybe because something to do with Rogers death or maybe because they had done something to him in the past.
Shiki Vs. Roger: