All Five Elders have Yokai Devil Fruits
With Saint Jaygarcia Saturn showing his hybrid zoan form, he did so in a pentagram, HOWEVER, this pentagram isn’t actually a pentagram, but a representation of the 5 elements/5 planets.
Each of the 5 elements originated from the 5 visible planets, the ones that the Gorosei are named after.
Each of the elements corresponds with a planet. In fact, the first phrase of each represents a day as well. We’ll get back to that.
Let’s look at Saturn, who represents the element of Earth. The theory kind of hinges on on thing: he’s actually a Tsuchigumo and NOT an Ushi-Oni. The Tsuchigumo is a pretty legendary yokai and has a distinct and clear representation of earth/soil based on the mythology.
If Saturn’s Devil Fruit is a yokai that represents earth, then I suspect that the other four Elder Stars represent yokai representing their elements as well, but based on the GODAI, not the Wuxing. Here are some guesses:
Jupiter (Water, 4) – Umibōzu, a paranormal phenomenon or yōkai from Japanese folklore.
Mars (Fire, #3) – Basan, a fowl-like bird with origins stemming from Japanese mythology and folklore.
Venus (Wind, #2) – Fujin, the Japanese god of the wind.
Honestly this one is a big stretch. That might actually just be Venus’ awakening smoke and he has some other wind-based form like the Kaze no Kami (who can become intangible… something of interest given Zoro’s hinting at the future ability to cut nothing).
And then Mercury (Void, #1) – Baku, a supernatural being that is said to devour nightmares.
The thing about the void is it represents spirit, thought, and ideation and the Baku is a creature that removes nightmares.
*Theory by the_idiotlord