By the same token, an immature cat isn't a kitty, it's a kitten. An immature rabbit is not a bunny, it's a kit. The terms "bunny" and "kitty" are just as attributable to adult members of the species; it's just a cutesy diminutive.

If you're going to generalize from one example, then why is it that Joyce references a "moo-cow," but never mentions a "quack-duck" or a "trumpet-elephant"?