I'm just throwing this out there, and yeah…it's probably going to sound stupid.
What if Kaido really is just...a creature; a massive creature? Like a large serpentine beast, perhaps a giant sea creature. Perhaps it's been...tamed by someone, a creature tamer of sorts, and they control it (but they, themselves, are pretty weak).
It could also be a giant mechanical, though life-like, creature. A weaker person inside could control it (like a mech, Oda always said he wanted to do a large-scale mecha series afterall; instead doing One Piece, though such tech is possible in the universe).
–- Update From New Post Merge ---
@DARK_RITUAL:
You still don't understand the kind of hype Oda put on Kaido right now?
You still don't understand his title is strongest creature IN THE WORLD, and not in all of existence right?