I didn't read the spoilers and don't plan on it, but the mythology has it that a koi that swims to the top of the waterfall using willpower and strength becomes a dragon as the gods reward it's perserverance. It's the reason behind Kiryu being "Like a Dragon" and having the dragon tattoo, and Nishiki having the koi on his back, as he wants to become a dragon, while Kiryu is, and the dragon Koi myth reflects that.
Kaidou represents the fish probably, that he kept swimming up the waterfall and through willpower and patience, he became a dragon. Big Mom meanwhil weas born strong.