He's a 5 year old unneutered Golden Retriever.
We were away and my FIL was dog sitting. One day DDog was wet so FIL dried him with a towel and noticed it and he's just pointed it out to me.
It's in the middle of his back and is roughly the size of the top of my little finger. It doesn't seem to hurt him. I groom him every few days and haven't noticed it before.
Any ideas on what this might be? I'll be getting a vet appointment tomorrow.