Dragons are by far one of the most interesting thing in One Piece. Especially the mystery behind them since they're ALL connected somehow. If it isn't already a priority to you, here's a quick thread why it should be

The first (non-filler) Dragon we see is the one on Punk Hazard. But before that, let's start with the first mention of dragons. The first mention was in Thriller bark when we were told Ryuma slayed a Dragon. We find out later that he slayed a WESTERN dragon

The interesting part is, the first time we SEE a dragon is in Punk Hazard. A western dragon. The 2 dragons were part of a series of experiments by vegapunk and the world government to produce dragons. We don't know why. In total there were at least 13 dragon models

For some reason the world government was really determined to create dragons. We are also told by brownbeard they LOVED the one on PH to the point they named it. For some reason, the top heads of the WG are called "celestial dragons"

For a reason unknown, they also like to "mark" their slaves with a brand called "The hoof of the flying dragon" which coincidentally also belongs to a western dragon

This is where it gets interesting. Wano is an isolated country and Ryuma was said to fight off invaders from the outside world, presumably the World Government and THAT'S when he slayed the dragon, showing the world the power of samurai.

This implies SOMEONE (WG) from the outside world came with dragon(s) to wano in hopes of invading it. This shows to me that in ancient history, the celestial dragons DID have LIVING western dragons as pets and might have used that against the world, which ties back to why-

They loved the western dragons from Punk Hazard so much. Perhaps the celestial dragons forced vegapunk to create them. Vegapunk also created the artificial dragon devil fruit which is identical to Kaido's, both with heavy relations to wano

Now there's even MORE references to dragons. We know from chapter 957 Garp defended the celestial dragons along with Roger. He now hates them, but for a reason unknown, he also named his son Dragon, ironically the one that wants to bring down the celestial dragons

And very very suspiciously in chapter 905, Stelly asks Garp about his connections to the celestial dragons because he wants to BECOME one is garp that close to them? We also know sabo uses the dragon claw fist, which we assume is taught by Dragon himself

The strongest creature in the world is a dragon, and the most dangerous man alive fights using Dragon martial arts. Theres an insane level of untapped power in dragons and the WG probably wanted to capitalise on this

There's a lot of important stuff roaming in the background. Oda has an ENTIRE notebook in his office DEDICATED to just dragons. His friend and former assistant hiro Mashima has an entire series based on dragons (Fairy Tail)

Even in The infamous One Piece magazine vol. 5 Oda specifically made a section discussing dragons and included THIS. Implying there's something revolutionary (pun intended) about dragons is yet to be known.
*BLACK BOX is a phrase for something unkown

THEORY TIME: the Celestial dragons are mad they no longer have their dragon army. Monkey D. Dragon was a human weapon by the celestial dragons in an attempt to replace the dragons. They urged Garp, their strongest warrior to have a child for this purpose

Garp eventually understood the kind of scum the Celestial Dragons are, and secretly helped Dragon escape them. (they are still in contact) The celestials were upset so they turned to artifical dragon making by vegapunk. Which gave us the prototypes

However, the research never stopped and Vegapunk FINALLY completed or restored a dragon(s) to its former glory. And coincidentally its at the same time as the avolshiment of the warlord system. This new completed dragon by the science unit is called the S. S. G.