In chapter 996 Yamato began what appeared to be a transformation, gaining sharp fangs, before being interrupted.
My idea is that Yamato ate the mythical zoan-type devil fruit Neko Neko no Mi, Model: White Tiger.
In chinese mythology, Dragons have a mortal enemy, the Tiger. Tigers were said to be on equal footing with Dragons, and that they were similar beings in terms of strength and will, but very different when it comes to values.
Many people also pointed out that Kanjuro’s drawing of Ryunosuke was a hint at Kaido’s dragon form, but after that, what did he draw?
A tiger, on top of the dragon, as if they were fighting. Yamato has a Tiger Zoan Devil Fruit (maybe even a mythical tiger) and he will be integral to taking down Kaido.
*Theory by PowderMonkeyTick