Well is it bald? Or is it just a callus?? Sometimes dogs, older ones more typically, will get a callus on their elbows. Sometimes one elbow, if the dog is prone to laying on that particular elbow when it goes down...
If it looks like a callus, I wouldn't be too worried about it as I'm pretty sure there's not much you can do about it but maybe put some balm or cream on it so its not very rough to the touch... But I don't think the fur will ever grow back...
If its something like a hotspot, where the skin is irritated and itchy you can get some Bag Balm and rub that into it. When my old dog Bean got hotspots, Bag Balm always made it more bearable for her until it went away.
Hope its not anything to be concerned over, but if you wanna post pictures, I'll be able to tell you if its a callus or not.
