It was revealed that Kaido ate the Fish Fish Fruit, Model: Azure Dragon 38 years ago during the God Valley Incident. Charlotte Linlin gave this fruit to Kaido, who at the time was her crewmate on the Rocks Pirates. Big Mom considers this a debt for life that Kaido cannot escape from.
According to One Piece Magazine Vol.13, this fruit is a Mythical Zoan and it allows Kaido to transform into an Azure Dragon at will, and also command fire, lightning and wind. Among the Zoan types, it ranks among the very best.
The ability to transform into an Azure Dragon, enabling a wide variety of out-of-the-ordinary attacks at the level of causing natural disasters. The breath that’s exhaled in dragon form sometimes turns into flames so hot that they consume buildings, and sometimes into whirlwinds so sharp and powerful that they sever the human body.
The roaring yell becomes thunderous lightning that reverberates, and the strong winds that are created by spinning the body create countless tornadoes that cause extensive damage. By grasping the Flame Clouds that are created in the sky, you can levitate through the air. You can cover objects with the Flame Clouds you create, and can also levitate objects around you.
The range within which the current user Kaido can levitate, is so vast that he can move an entire island with more than 35,000 people staying on it while floating. In his Human-Beast Form, he combines the maneuverability of his human form with the versatility of his dragon form. It is a very well balanced form suitable for combat, and is used frequently in serious battles.
While this fruit is classed as Mythical Zoan, with dragons explicitly stated as having mythical status in the One Piece world, dragons have also been real existences in the series, although modeled after Western dragons.
The Azure Dragon, in Chinese and other East Asian mythologies, is a Dragon God and one of the Four Symbols (the “Four Guardians”) among the Chinese constellations, representing the cardinal direction of the East.
With its “fish” designation, this fruit references the connections present between dragons and fish in East Asian (especially Chinese) mythology. Particularly, the fruit may draw inspiration from the Chinese Dragon Gate myth, wherein the Dragon Gate is located atop a waterfall cascading from a legendary mountain, on the Yellow River. Many carp swim upstream against the river’s strong current, but few are capable or brave enough for the final leap over the fall. If a carp successfully makes the jump, it is transformed into a majestic dragon; thus, a Chinese dragon’s large, conspicuous scales indicate its origin from carp.
According to Oda, Roronoa Zoro would have this Devil Fruit if he was allowed one, albeit “consumed” by one of his swords.
The translation from One Piece Magazine Vol.13 by strawhatmaterial (https://www.reddit.com/user/strawhatmaterial)