The concept of a spirit bunny, or any kind of spiritual figure associated with rabbits, is not part of the book or the film.
Watership Down focuses on the story of a group of rabbits escaping their warren and searching for a new home. The story is driven by their struggles for survival, the dangers they face, and their determination to build a new community.
It's possible you're thinking of another rabbit-related story or perhaps something from a different type of media. If you could provide more context or details about the spirit bunny you're asking about, I might be able to help you further.