There seems to be a misconception about how zoan powers work. It may look like they're inherently limited to three forms, be them base, animal and hybrid, but this more than likely isn't the case for experienced users. A good example here is Onigumo, the user of a spider zoan that was able to partially transform when he grew additional arachnid arms and only that in Marineford. We have no confirmation of what his fruit is (we can only be certain that he ate a spider zoan of some kind), to be fair, so he may have eaten a mythical model; but I think he's just a regular zoan. Pekoms is another example, since he can transform only his back into a tortoise shell without going full hybrid. Other characters have shown abnormal hybrids too, like Bian, the Boa sisters or Epoida.
Now, regarding mythical zoan, it is true that they seem able to transform partially as a regular basis. Not only Marco was capable of transforming only his arms into wings or his legs; Orochi himself, who doesn't seem to be the strongest guy out there, can generate his orochi heads at will. The best example of this was when he grew two snake heads and nothing more while talking to kid Kanjuro. But the existence of cases like Onigumo and Pekoms basically points to this being an ability that all type of zoans can achieve.
I personally don't think that mythical fruits lack a proper hybrid form, though. I'm quite sure that what we saw in Marineford was Sengoku's hybrid, there's no way that was the animal form because it maintained many traits of human Sengoku and lacked other traits from a full daibutsu. So I'm expecting Kaido to go hybrid and even awakened, with certain moments of partial transformations.
