The koi carp symbolism at the beginning of Wano that associates with Kaido becoming the strongest creature and fighting to the top of the figurative waterfall at least would be for waste if Kaido isn't actually a fish in some form. It would be the ultimate tale of survival and ascendance, the fish who became a god like in the mythology/folklore.
Plus I don't think these guys were included by coincidence in OP, especially in Dressrosa an arc where a subordinate/business partner of Kaido was ruling and had a coliseum for gladitorial battles and themes of passion, violence/bloodsport, fighting in general and such:
Look at Kaidos left arm too, that is NOT from his DF:
Kaido is a BLUE/AZURE dragon so why does have those scales on his arm, which seems to be tattoos most likely but still? There's still a lot we need to learn about him.
Kaido symbolically is at least that metaphorical fish who climbed that waterfall and ascended to the heavens, the one who was granted divine power and status by the gods for doing the impossible and reaching the very top, the highest summit no others could achieve, in other words, The World's Strongest Creature and currently living being!
I genuinely doubt anyone alive is
physically and mentally stronger than Kaido right now, maybe even dead too. Look at how much spotlight Oda is giving Kaido and how many times he made Luffy lose to him and how many powerups he's given to Luffy in a vain attempt to even let Luffy
try and fight equally to him!
There's a reason Kaido was prevented from going to Marineford, it would have been total annihilation! Even more than what WB and BB both did to it! It would have been the most bloodiest, violent, destructive rampage ever, especially if Kaido saw what happened to WB and how he died, that betrayal and also if he was trying to save Ace too as some people believe including me!
Marco the One Piece Theorist had some great theories about Kaido too, that align with similar ideas I had for him too: