Xena Parker 
Zoan devil fruits have some traits that make them unique. There is the obvious boost in strength, speed and overall senses of the user, but what makes them unique is their transformations. The transformations that zoan’s undergo is what makes them different from the other types of devil fruits. Typically the Zoan transformations are divided into two, the Man-Beast form and the, Full on-Beast form. This is trait that will be directly connected to the type of animal/creature that is tied in to ones devil fruit. Other transformations can be added onto the base ones, as we see with Kaido’s purple form and Lucci’s skinny form.
On top of having regular Zoan devil fruits, there are other two categories that were introduced. That is, Ancient Zoans and Mythical Zoans. The transformations still apply to them, its that they have unique creatures or mythical entities within them bringing about very cool transformations.
During the raid in Onigashima, it was revealed that Luffy’s Devil fruit wasn’t a Paramecia but a Mythical Zoan. Specifically the Hito Hito no mi: Model-NIKA. This made the fandom split in two since the reveal made us look at Luffy’s journey differently. Were all of Luffy’s victories already predetermined, since he bears a fruit connected to god. But what I was asking was, since we now know that Luffy is a Zoan user, and that Gear 5 is his full Beast form, what is his Man-Beast form????
He sure had already unlocked it since the Man-Beast form is derived from the fusion of the user, and the devil fruit right in the middle. It gives the user the power of the full-Beast form, without sacrificing mobility or speed as most of the full on Beast forms are huge. Looking back, Luffy has 3 transformations already under his belt, technically one. Gear 2 and Gear 3 are more of a body doping transformation, since all Luffy does is increase his blood flow and his body mass(respectively) to use them.
On top of that, you Man-beast form needs to show some of the power that you use in your full-Beast form. That is not present in the first two transformations, but it is in the third one, Gear 4. People tend to forget how similar Gear 4 is to Gear 5. All the sub-transformations in Gear 4, Bound man, Snake man and Tank man, are directly derive from Gear 5. From the infinite stretching of Luffy’s Limbs, the smoke cloud, to the ridiculous ways that Gear 4th manifested itself as, if Gear 5 is the power to make imagination a reality, then Gear 4th is the best display of that. But how did Luffy achieve this, he didn’t even know what his devil fruit’s real category was.
I do believe that it is connected to his Haki. Luffy goes into his Gear 4th transformations by inflating his Haki, and I think that’s how he was able to unknowingly, unlock his Man-Beast form. Since Haki is the users spirit, I believe that when Luffy inflates his Haki, he triggers Gear 5th, but since he doesn’t know about his Devil fruit, it stops him from fully awakening. I said awakening since Luffy’s devil fruit is in some ways very similar to Yamato’s, in that they don’t have an normal Beast form since it’s already awakened.
I believe that the smoke cloud that Luffy has when he is in Gear 4, combined with the spike in ridiculousness and power, is a clear indication that this is his, Man-Beast form. That, or Oda is trolling us, as he always does. Thanks for reading the article, and don’t forget to comment on what you think is Luffy’s Man- Beast form.
