The Wano arc in the One Piece has been nothing short of a crazy thrill side since its beginning chapters. Zoro being wanted murder, giant samurai baboons, and even Luffy "using" a cursed sword to fight! Even crazier is that these few examples don't even scratch the tip of the iceberg. Probably one of the biggest moments of the arc was actually the Kozuki Oden flashback, which might also be the most important one the series has ever had. This is because it reveals much about the man who's responsible for starting the entire story of One Piece, Gol D. Roger, and even gives insight into the titular treasure fans have waited years to see.
Since the Fishman Island arc, the character of Joyboy has been one of the series's biggest mysteries. Joyboy is the name or title of a character of great importance from the Void Century, a period of history that spans 100 years that has been erased from history. He was so important that he was able to commission at least one poneglyph to be made specifically for him to use as an apology letter to the fishmen and merfolk. Why he was so important and what his full role in the world was though is still a mystery.
That being said, Joyboy's story might not end in the Void Century. Kaido and his right-hand man King, as well as Roger, have all mentioned or implied that Joyboy is going to make a return at some point in time, with Roger and Oden believing it to be roughly around where the story currently is. King even believed that Joyboy has already "returned" as Kaido, and decided to follow him partially because of this. Due to how long it has been since the Void Century, many people both in One Piece and in the fanbase have assumed that Joyboy's "return" is closer to reincarnation than anything else, but what if that idea needs to be taken much more literally?
Since the Zou arc, fans have known about the giant elephant Zunesha wandering the seas carrying the Minks' homeland on its back. It seemed like it was just wandering aimlessly, but this idea was very soon debunked. According to Zunesha itself, its trek is actually some form of punishment. It committed some sort of crime during the Void Century and as a result, it has been sentenced to walk for eternity, being unable to act or even protect itself without being explicitly told to do so. By the end of the arc, readers/viewers are even told that Zunesha might actually be walking the seas specifically searching for something or somewhere.
Given what we know about Joyboy and Zunesha, it is actually quite possible that they are one and the same. Both hail from the Void Century, but there is way more than just that one similarity. They have also both done something wrong, with Joyboy not fulfilling his promise to Fishman Island and Zunesha's being a mystery, but still relevant.
The current World Government also felt the need to completely eradicate all the evidence of the Ancient Kingdom Joyboy was from as well as his very existence. Whoever stopped Joyboy and defeated the Ancient Kingdom probably also sentenced Zunesha, as they would have to be quite strong indeed as the elephant is easily one of the most powerful creatures in all of One Piece.
There is also the matter of the secret Devil Fruit mentioned by the Gorosei in recent chapters. They claim that there is a Devil Fruit that had an awakening so dangerous that they erased its true name from history. This is important to note as not only did this dialogue appear as Zunesha arrived at the coast of Wano (possibly implying a connection between the two), but also because this kind of erasure is very similar to both Joyboy and Zunesha. This Devil Fruit matters to the theory because it might be what connects all three together.
What if Joyboy ate a Devil Fruit that allowed him to turn into Zunesha? As strange as it sounds, it is not impossible. Fans know that anyone that eats a Devil Fruit immediately learns the name of it. This makes the popular theory that the Gorosei were referring to Luffy's fruit much less plausible. What if the Gorosei erased the fruit's name from history as it could be used to discover more about Joyboy?
What really ties everything together is the prophecy of Oden that states Joyboy will return in some form and come to Wano, which is why it needed its borders opened. Whatever the One Piece is, it led Oden and Roger to believe that in roughly 20 years Joyboy would return, and somehow Wano is crucial to that return. It is implied that this prophecy is partially why Kaido is so interested in Wano as well. If he believed he was Joyboy, then maybe he thought by conquering it that feat would prove that he is Joyboy.
The only real evidence that could disprove this theory is that Momonosuke recently called Zunesha Joyboy's companion, but that doesn't necessarily have to be the case. In chapter 1041 Momonosuke also stated that there are pages missing from the journal where Momonosuke got this information, meaning that the information he has is incomplete. This might have caused him to misinterpret the connection between the two, especially as he admits that the missing page held the most crucial information.
That being said, there is still much more evidence that leans towards this theory being correct, and at the very least also has enough evidence that proves a connection between the two. While the Devil Fruit connection is admittedly a bit shaky, a being from the Void Century has made its way to Wano just in time to fit the prophecy. While many believe Joyboy to be Luffy or even Momonosuke, Zunesha is still a strong contender.
