That makes the Klabautermann lore even more mysterious. Ratatoskr being an animal is one thing, but already starting off as a Spirit. I mean how did it even come into existence in the first place?
This would all go back to the old discussion about real magic existing in the one-piece world.
Klabautermann are kind of curious, with this line of thinking. They kind of fit and they kind of don't. Because most ships DO have a lot of metal as part of their makeup, like nails and beams and whatnot. But, mostly they're made of wood. And, I don't know if you know this or not, but wood is not a metal. Even if we assume some wood has slightly "metallic properties", like maybe Adam Wood could, not all ships are made of Adam Wood. The Merry was just made of ordinary wood, as far as we know. So, that doesn't quite fit with the other examples of metal somehow being important to the process.
My gut says that Klabautermann are related to this process, but not quite the same. "Cousins" as it were.
In my head, the difference is how many people are "engraving" their souls into the object.
Most of the other examples of objects with "souls" inside of them only involve ONE strong willed person "engraving" their soul into the object. One person, "engraving" one soul, into one object.
But, Klabautermann are different just due to the fact that the entire crew of the ship is involved in the process. Every member of the crew puts a little piece of their heart and soul into the same object, together. Maybe because they're all working together, it allows them to "engrave" their souls into objects that aren't metal?
If it's just one person working by themselves, they NEED to use a "hard" object like metal in order to keep their soul inside the object.
But, if a bunch of people work together, all the different "soul pieces" end up helping to keep each other together, knitting together in a way, so they can be transferred into a "softer" object, like wood. Each soul "holding on" to each other.
It could basically be another form of "The Power of Love" that we keep hearing about in the story. From what I can tell so far, "Love" requires multiple people to work together towards the same goal. "Group Haki" is the way that I think about it personally. So, Klabautermann would be a natural product of that "Love". A "child" born from a crew's "Love" for each other, and their ship. A combination of all their souls working together, towards a common goal. A manifestation of their shared dream.
...Which is SUPER CHEESY, I'll admit. But, I think something along these lines is possibly what Oda is going for. Hopefully, we see Klabautermann come back into the story, eventually. (Which isn't impossible, if The Man Marked By Flames turns out to be Roger's ship the Oro Jackson, like I think it could be. But, that's a theory for another time.)
Part of me wonders if Seastone could also be involved in this?
Now, Seastone isn't necessarily a metal. But, it does get TREATED a lot like a metal, in most cases. It's exceptionally hard. It's used in objects that you would expect to be made of metal, like handcuffs and weapons and even the undersides of Marine ships. So, at the very least, I think it can work as a form of metal for this process.
So, it's not impossible that Seastone has it's effects on Devil Fruit users, because all Seastone is "engraved" with the soul of someone or possibly a group of people. The power to weaken Devil Fruit Users may come from some "spirit" within the metal. Maybe the Sea God? Or maybe the Dragon race? Or maybe some sort of Demon King, like Tot Musica? Someone with a grudge against Devil Fruit Users, who would want to hurt or control them.
Whoever it is, it's probably someone or something "imprisoned" in the center of the planet. Because one thing that's important about Seastone is that it's released from volcanoes. So, my pet theory is, something is frozen in the center of the planet. After the "Forbidden Sun" was mined, something was imprisoned in the empty "quarry" that got left behind, and frozen. And the "steam" from them slowly unthawing is what's "infecting" rocks and minerals and metals with their properties, turning them into Seastone.
I don't know, stuff to think about. Again, I could be on the wrong track with ALL of this. But, there's enough going on with souls and metal that I think Oda is going to end up revealing something along these lines.