Hi,
in my opinion, form is a view, but in most of examples of Zend_Form it is a model or a controller. Why?

Those examples you have seen are intended to foster reusability hence Zend_Form is moved into the Model. Besides that the concept of having Thin controller, Fat Model is also applied.

An Example Zend Framework Blog Application - Part 2: The MVC Application Architecture - Maugrim The Reaper's Blog

read the section on Controller vs Models for a better understanding.

Cheers