I think I've only seen the Tol'vir archeology mount once so far, so that seems pretty rare. Otherwise, I'm on a highly populated, older server, so you see all kinds of rare mounts. But, I'd say most people don't know the difference between Halaa and Kurenai mounts, so they're not really that special. Most of the mounts that are just different skins aren't really that noticeable either, but something like the Ashes mount is pretty rare and unique. Also, I don't see the white chicken very often, or the big rooster, and I've yet to see a rhino.