Cat got in a fight, now has a swollen tail lump. What to do?

Thank you for submitting your question regarding Rilian. He will need to see his veterinarian as he most likely has developed an abscess under his skin. Cat teeth are very sharp and puncture the skin introducing bacteria deep inside. The overlying skin heals quickly trapping the infection underneath. A visible swelling will develop. This is what I suspect has happened to Rilian. Most cats will develop a fever and may stop eating. He will likely need antibiotics and possibly to have the abscess lanced. In the meantime, it is fine to shave the overlying hair. I would not apply anything topically aside from water to cleanse the wound though. You can apply warm compresses for 5 minutes 3 times per day if he will tolerate it as well. I hope this information helps!