With Big Mom calling him a thing and the World Government labelling him the strongest creature, Kaido is a World Government experiment on the loose.

I feel the World Government has been trying to create super soldiers for the longest time and this experiment was first successful with Kaido an indestructible soldier created by Vegapunk.

Kaido lived his life as a criminal, so obviously he was captured by the Marines as a test subject for the giantification project of the World Government. Lots of failure and dead test subjects until they finally come across with Kaido and miraculously come up with the optimal solution and procedures for the operation. But he had free will, so he escaped and became the terror he is today.

Kaido destroyed everything on the laboratory, every record, and turned Vegapunk back to square one again as he left the ruined laboratory. This is why the next super soldier was Kuma, but not to have the same mistake happen, they wiped his mind and made him a robot.

This is a total failure for Vegapunk. As usual, it was all covered up by the World Government. But the research team is still recovering the perfect formula for giantification.

The same reason why Caesar Clown is so eager to perfect the art of making human giants and eventually gain his lost reputation and become the greatest scientist.

But Caesar Clown is a bit different this time; he got this wonderful idea that can possibly tame a beast like Kaido if he gradually ingests some addictive drug known as NCH10. This way, Caesar Clown is certain that his human giants will be obedient and come back for more without taking it by force just like what Marines are doing.