User blog comment:The big croc/Other Zoan transformations/@comment-4493594-20111121131238

It was only due to the rumble ball that Chopper knew the other forms at all. So unless some other Zoan type has access to a drug similar to the rumble ball, Chopper will remain the only Zoan type with more than 3 transformations. I don't believe you can simply "learn" other transformations.