I never posted my theory here, so, I copypaste my own stuff from reddit. You all remember Moombas from FF VIII, right?

"Calling it now - we were all blind. Happy Bunnies are obviously male Viera. That's why no one has seen them before, no one except such a form. When Viera come of age and are male, they turn into Happy Bunnies. Think of them similar as of the Moombas in FF VIII which were also another evolution state of a completely different race."