Does he wear egg bar shoes behind? Those can really help. I second gabapentin, as well.
Sorry to say, I think you need to consider euthanasia. The horse is on three legs, attacking other horses, loosing weight, you say he is in pain, he is miserable and you appear to have tried everything and he is 18 years old. He is in PAIN and has been for 30 days.
Given his prlonged lameness, no possibility of cure, advanced age, and very poor quality of life, I would euthanize. Like, I would do it this week. If I were unsure though, my question to the vet would be: are there any treatments available that we can reasonably expect to provide all-day, every-day pain relief? I hope you won’t settle for just “improvement”, because even 50% better than the status quo (based on what you describe) would still meet my threshold for euthanasia.
