So far im leaning toward an actual dragon.
That post rox in one of the times he captured was experimented on by vegapunk and they took his dragon essence to make momos smile fruit.
Another side thought maybe to make smile you need to use the essence of the actual animal in that smile. Which is why they are all standard animals ( youll know more than me on what animals the smiles are) instead of using the fruits of his all stars and tobi roppo and why the gifters arent part dinosaur/ mammoth etc and just normal animals.
My problems with the idea of Kaido being an actual dragon are:
1) If Yamato is a dragon in some way too it will harm the connections between faterhs and children, since Momonosuke is Oden's actual son yet ate the dragon fruit and Yamato is Kaido's actual son yet wants to emulate Oden. It feels a bit disbalancing for Yamato to be a dragon too and not just a "traitor oni".
2) Human Kaido still has something weird about him because of the horns. We've been consistently seeing some very tall individuals, even above giants in the case of the Numbers, who have those ogre-like horns. And Kaido's don't come from his dragon self since Oda's dragons never have them, so they are likely original of his human appearance.
3) For a guy who wants to build the strongest crew out of devil fruit users I have problems seein him as a non-user, and I doubt he ate the mythical Oni fruit or something because then it would be the lamest design from Oda.
4) Considering that, so far, five out of his nine top subordinates ate models of the Ryu Ryu fruit (potentially all of them bar Jack if Hyogoro's comment about dinosaurs end up being literal) I'd find really weird that Kaido himself didn't eat the mythical model of this family. It's just a very organic hierarchy and I don't see Oda breaking it.
Regarding how Vegapunk created Momonosuke's fruit, maybe he had the mythical model of the Ryu Ryu (which Kaido would end up eating) and tried to replicate it. In this scenario, Caesar would need to work with genetic material because he wouldn't have the real models of each fruit to directly analyze their constitution before being eaten and therefore he would have a harder time recreating them, just like if I were given the pieces of an old phone and I had to create a new, modern one but you had access to a very intrincate and advanced phone and tried to replicate it knowing how the final result is (I'm using this comparison because I'm pretty convinced that "natural devil fruits" were also artificially created during the Void Century).