I have a Barred Rock hen with the same thing! It's not in the same place, but it looks EXACTLY like that. I did tie it off, but it didn't work. I'm going to examine closely again and band it next time. I really hope someone can identify this. I haven't had much luck finding information. Researching about chicken tumors is horrifying, but it really seems more like a cyst anyway. Feather follicle cysts seem to be the most common, but not this big. Still possible, I suppose.