It's an unfortunate consequence of the broken aliasing system. "fang" is aliased to "oerba_yun_fang" so every time an image with the tag "fang" is uploaded, mostly by the bot, it's changed to "oerba_yun_fang". unfortunately this alias cannot be reversed.
The only solution to the problem I can see is just let the Final Fantasy character have both "fang" and "oerba_yun_fang" and use "cute_fang" to denote that little fang that some characters have sticking out, and obviously when they have two fangs like a vampire the tag would be "fangs".