May sound crazy, but what if Kaido isn’t an Oni or human who ate a Mythical Zoan-type Devil Fruit but is an actual dragon from the past? I believe a lot of things would fit into my theory.
Kaido has never been called human but only “creature” or “thing”. What if Kaido has been captured by the World Government a long time ago (800 years ago?) and they have been doing experiments on him?
It is said that some dragons are immortal so they could do whatever they want and they wouldn’t die. Especially Vegapunk used to work on these experiments and could extract his DNA to create the artificial Dragon Devil Fruit (Momonosuke’s Devil Fruit).
Also other dragons could be cloned from him but they looked a little different than Kaido (dragons from Punk Hazard). Several times Kaido could escape but got captured from time to time and had to break free again.
Some dragons have the power to turn human to be able to hide. Since Kaido is a huge dragon he also is huge as a human person (Momonosuke also is just a small dragon depending on his human size). It also would make sense why Kaido is the most dangerous creature on land, sea and in the sky. Since he is a real dragon and not a Devil Fruit user he won’t die or lose his powers in water.
The experiment the World Government has been doing on him made him insane and he just wants to forget everything and die. Newgate was the only person to challenge him and being able to kill him. We don’t know how Shanks could stop him from attacking Whitebeard during/before Marineford War, so we can’t tell if Sanks is considered to be strong enough by Kaido (guess not).
This is why Kaido was collecting Devil Fruit users for his crew. He wants to destroy the whole world because he doesn’t see any other solution to die (this also could be the reason why Toki traveled to these different times 20 years ago and now. She may be from 800 years ago or maybe even before that time and has seen the future. She also has seen that she needed to give birth to her children, so they could stop the destruction of the world with the Straw Hats).
Maybe my theory is total nonsense or maybe there is something that could be true, but I just wanted to write it down and hear your opinions. So, what do you think?
*Theory by Shishi Sonson