I am working on creating a sapient reptilian species from a mineral-rich Planet that has very little flora. I was wanting this species to use a super acid (specifically fluoroantimonic acid) excreted through their tails for hunting and for defense. How would this be plausible?
Not With A Current Understanding of Biology
Fluoroantimonic acid reacts with almost all organic compounds making it completely inimical to life as we understand it. It will spontaneously explode or combust when in contact with the slightest trace of moisture. Even going to the old hand wavium scifi plot device of silicon based organisms doesn't work either since it reacts violently with that too. I can see no plausible way that the chemical reactions required to synthesize this acid could take place within an organisms body, and no way that said organism could ever safely store said acid within its self, nor deploy this acid without harming itself.
Stomach acid is pretty nasty, and we (life on earth) hold it in place using sacrificial tissue and protective secretions. That wouldn't cut it in your case unless your creature was metabolising rocks as part of its diet.. which given the lack of flora sounds reasonable. A creature like that might be able to grow a sack of solidified rock coated in secretion. Whether such a creature could move much.. is a different question.