I was looking into classic Japanese folklore and fairy tales to examine the parallels they share with the current Wano arc, and I came across some very interesting stuff.
This theory will be divided into two sections: Japanese Folklore and Devil Fruits.
My first theory is that Kaido, Yamato and Black Maria are all the same race. The Oni or Yokai Race (Oni are a type of Yokai). Yōkai are a class of supernatural monsters, spirits and demons from Japanese folklore.
Looking at Japanese classical folklore, we can see that the Oni are portrayed as the enemies of “peach boy Momotaro”, whom Momonsuke is based on.
We also know that Big Mom, has three races missing in her crew, one of which is King (who is a Tengu), Giants, and… Oni/Yokai. In addition, Kaido is always referred to as a “thing” or “creature”. Kaido’s, Maria’s, and Yamato’s horns are clearly real horns, unlike the others in the crew.
I also believe that Black Maria is Yamato’s mother. There are many different Oni in Japanese folktale, I believe that Black Maria is a “Yama-Uba”, a type of yōkai found in Japanese folklore. Yamauba is a type of “evil old hag” monster that devours little children after acting nice to them (we saw how Black Maria reacted to Momonosuke being beaten and hurt).
Sometimes they are referred to as “Onibaba”. In addition, they usually take the form of a women in their twenties, known as “yamahime”, and a “yamahime” would wash her hair and sing in a lovely voice (we saw Black Maria in the recent chapters acting like this).
Okay you may be asking why I think she is Yamato’s mother?
In a variation of Yamauba’s story, there exists a story where Yamauba is the mother of “Sakata no Kintoki or Kintaro”, another famous Japanese folklore, a child of superhuman strength, who was raised by a yama-uba. The yama-uba was Kintarō’s mother, impregnated by a clap of thunder sent from a red dragon. We know that Kaido is a dragon that uses thunderclap attacks!
In the story of Kintaro, he met the samurai Minamoto no Yorimitsu as he passed through the area around Mt. Kintoki. Yorimitsu was impressed by Kintarō’s enormous strength, so he took him as one of his personal retainers to live with him in Kyoto. Kintarō changed his name to Sakata no Kintoki. This is very similar to Yamato and Oden, while Yamato didn’t become a retainer, she wished to be like Oden! Another interesting fact, on Children’s day in Japan, parents put up a Kintarō doll in the hopes that they have boys who grow up to be strong, WHICH MAY EXPLAIN WHY KAIDO CALLS YAMATO HIS “SON”.
I also think their names are a play on these folklore characters
(Kai)do = Yo(Kai)
Yamato = (Yama)uba
As for Black Maria = this is where it gets messy and interesting. Kin, in Kintaro, also means gold, which may be a play of Maria’s hair that we saw on the volume cover. Also, the name Maria means “sea of bitterness”, and her horns are red. Her horns being red are done purposefully by Oda >> in Japanese theater, there is something called a “Hannya”, a red mask representing a jealous and bitter female demon (which we saw Yamato wearing in her intro). Maria means bitterness. Also, Hannya’s eat beautiful ladies (maybe next chapter?) Black Maria is also the “queen” in a deck of cards.
To sum the first theory up, Kaido, Yamato, and Black Maria are all the same race. With heavy influences and plays from classical Japanese culture.
Yes, I know the first part was long as hell but bear with me.
How does this tie in to my second theory about Devil Fruits?