This is a somewhat short theory that I came up with after wondering why a certain pirate is called by one of his epithets. So, basically, this theory aims to hit two birds with one stone.
First off, what do we know about SAD? SAD is a unique substance created by Caesar Clown for the production of the artificial Zoan devil fruit, SMILE. Now, with the revelations made in the current arc, we now know the details on how SMILEs work. But how about SAD? Why is it that Oda chose to make an ingredient of SMILEs to have some sort of significance in the story? Why not just go with a combination of chemicals that Caesar discovered to create SMILEs? One reason I can think of is to highlight Caesar's method in creating artificial devil fruits as compared to Vegapunk's, who was also able to make at least one albeit a failure (the one Momo ate). Another possible reason is because SAD actually has a very special source.
As explained by Law in Chapter 698, SAD was made using the application of the "Lineage Factor" that was discovered by Vegapunk and Vinsmoke Judge.
Screen Shot 2020-07-08 at 7.20.36 PM.png
With the knowledge of Lineage Factors, one can take the biological blueprint of various animals and use them to perform scientific feats such as drastic body modifications, cloning, and creating devil fruits. We can see this in Caesar's SMILEs and Vegapunk's failed artificial devil fruit. With how Lineage Factors work, we can come up with a simple, logical assumption that a specific target source is important. For example, in order to create the (failed) devil fruit Vegapunk made, he would need the Lineage Factor of an actual dragon.
This then should also apply to Caesar's SMILEs. If he wants to create a "bat" SMILE, he needs a bat's Lineage Factor. If he wants to create a "horse" SMILE, he needs a horse's Lineage Factor. However, despite considering this, I feel that Caesar didn't directly follow this logic due to the presence of SAD. Instead of taking the Lineage Factors of different kinds of animals, he only had one special source, which resulted in the creation of the unique substance, SAD, and the production of SMILEs that can yield different kinds of animal transformation.
If that is the case, what then is the sole source?
My answer is Kaido.
In Chapter 664, Brownbeard revealed that Punk Hazard was a research facility where Vegapunk and Caesar performed human experiments on convicts.
Screen Shot 2020-07-08 at 11.15.34 PM.png
In Chapter 795, Kaido was described to have been captured by the Marines multiple times and subjected to a whole lot of torture and death sentences.
Screen Shot 2020-07-08 at 11.26.02 PM.png
These then bring up the possibility that Kaido was experimented on as well. Perhaps in an attempt to determine the reason behind Kaido's seemingly invincible body, Vegapunk and Caesar discovered that he has a very complex Lineage Factor that is compatible with different kinds of animals. Caesar then used and needed only his Lineage Factor to create devil fruits for different kinds of animals, and the "impurity" of the source explains the partial/weird transformations of SMILE users. Also, if this turns out to be true, this can be an explanation for one of Kaido's epithets, namely "Hundred Beast Kaido".
P.S. Can anyone enlighten me if the "Hundred Beast Kaido" epithet is actually official per translation? I haven't found any claims online that says it is, but I also haven't found anything that says it's strictly incorrect. :