I had a hen with a very bad wound on her tail nub. It looked like this chicken's wound. No amount of debriding, wound care and even antibiotics, made one iota of difference. She was in a lot of pain, as well. I finally decided this had to end and euthanized her. This is the hen. https://www.backyardchickens.com/articles/introducing-a-single-hen-to-an-existing-flock.71997/ It was so sad and depressing. I wouldn't be surprised if it was a cancerous tumor she had.To me it looks like a possible squamous cell cancerous tumor, but an ingrown feather cyst or fungus might be possible. A vet might be able to give you a diagnosis.