I’ve seen a lot of people taking the reveal of Kaido being an Oni to mean that this is a new fantasy race we’re just now being introduced to, but I think that at this point most of the evidence points to this not being the case. There are a number of things that I think strongly imply that the “Oni race” and the “Ancient Giant tribe” that Oars originates from are/were one and the same, with “Ancient Giant” being a name given to them in the present in the same way that the real life “Ancient Greeks” wouldn’t have considered themselves to be “Ancient Greeks”.
The first thing I’d want to direct people toward is the general design inspiration of the Ancient Giants we’ve been shown. Almost all of them have had a very obvious Oni motif, with many of them wearing the sort of leopard-pelt loincloth that Oni are typically depicted wearing. Nangi from the Beasts Pirates literally just looks like a totally vanilla pop-cultural depiction of an Oni, with curly hair, a thick beard, a wide nose, and small, straight horns with grooves in them (he also had blue skin in that Where’s Waldo illustration, which is one of the go-to colors for Oni).
In some respects the Ancient Giants we’ve seen are more Oni-like than Kaido, as they have traits like sharp fangs and brightly colored skin without any sort of Devil Fruit transformation. With this in mind, why would Oda include two Oni-inspired fantasy races in One Piece, and then make the actual Oni race be the less Oni-like out of the two? This wouldn’t be an issue of “oh there are a few kinds of Skyfolk who have different wing-shapes but live in the same general region and are more or less a part of the same fantasy race beyond that”, it’d be a really weird redundancy that I think is a bit unlike Oda.
The second thing I want to point out lies in the naming scheme we’ve seen for Kaido’s Numbers up until this point. With a couple of exceptions, most of them (the more Oni-looking ones funnily enough) have a name that follows this pattern: [Character for their respective numeral][Character for “Oni”]. The three we were introduced to on the Live Floor, Jaki, Juki, and Goki, have names that translate as “Four Oni”, “Ten Oni”, and “Five Oni” respectively.
Things really slide into place when you realize that the Numbers are all test subjects. Assuming they were originally inmates from Impel Down or something, these probably aren’t their given names, but rather new ones they received after being turned into their current selves by Vegapunk. With that in mind, why would the more successful-looking ones be given names that contain the word “Oni” if the Oni and Ancient Giants are separate groups, and this experiment was to re-create the latter and not the former? If this were the case you’d think their names would contain the word for giant (“kyojin”), but they don’t. All of this confusion would melt away however, if these groups turned out to be one and the same.
Thirdly, it could explain Kaido’s fixation on Wano, and on Ongashima in particular. Kaido is sort of a part of a trio along with Doflamingo and Orochi, who both used wrongful means to claim rulership of a country based on a long-expired sense of entitlement by birthright. It would make sense that Kaido, who’s at the top of this pyramid, has a similar relationship with the territory he rules over.
There have also been implications that the “country of ruffians” established by Oars is actually Wano (i.e. the different climates in each region could be explained by them having been distinct islands at one point). This is admittedly speculation on my part, but you have to admit this would make a lot of sense.
I also feel the need to mention that the name “Onigashima” is implied to be one that Kaido gave to the island after he set up his operation there. We know it has some sort of possible Ancient Giant connection due to that massive Skull (that’s way too big to belong to even Oars), so this is another thing that I would argue supports my theory.
Lastly, let’s address the elephant in the room regarding scale: if Kaido and Yamato are from the same fantasy race as Oars and his family, why are they so comparatively small? In my mind, this is as simple as them just.. having a generational gap between themselves and Kaido’s Oni ancestors.
Theory by Evangeli_On_Drugs (https://www.reddit.com/user/JoeyJoJoHQ)