Here I go with my people comparisons again, but if Cooper was a human, his right elbow looks like olecranon bursitis. It's an inflammation of the fluid sac (bursa) that is normally at the elbow and can be from hitting it, or sometimes it happens for no reason. I don't think it's particularly painful in people - just looks weird. Not sure if dogs get that or not, but I would think it's possible - might be called something else though.
Marshall Dillon - 18 months old Kaiser - 11 1/2 yrs old - At the Bridge Ray (Radar) - 8 yrs old - At the Bridge