Have any Zoan types other than Chopper gained the ability to transform into more than three things? Chopper can transform into 7 different things (not including monster point) with the rumble ball and after the timeskip he can transform into 7 more. But why are there no other Zoan types that have more than three transformations!
Chopper can transform into 7 different things (not including monster point) with the rumble ball.


You answered your own question right there. He's completely reliant on a drug that he developed in order to make the additional transformations. Before the timeskip he was able to do the original 3 transformations without the rumble ball, and an additional 4 with the rumble ball.

After the timeskip he no longer needs the rumble ball for all 7 original transformations. There is nothing canon that explains why. However with the fact that he is able to transform into Monster Point with only 1 rumble ball now hints that he has developed a permanent effect of some degree that allows this.

It is assumed at this point that Chopper could theoretically develop something for other Zoan-class users assuming the rumble ball didn't already work for them. Suffice to say that at this point no other known scientists have developed anything similar.