If we find we are getting many questions that are breed specific, then it is entirely possible that we might want to add a breed tag (along with the animal tag, like dog or cat or what have you). At this point, though, it would just further divide the small question pool we currently have, and in many cases, the specific breed of dog or cat or so on doesn't matter.
It is generally better, as Monica pointed out in a comment, to have fewer tags and add more as there is a specific need for them - basically, letting them develop organically rather than forcing them unnecessarily. Part of being a good Internet information source and a good Q and A site is making that information easy to find, after all! :)