A Callus. This is a callus. Most dogs that sleep on hard surfaces, such as cement, wood, or tile, will develop calluses over their elbows, hocks or any other bony areas. Large breed dogs and overweight dogs are especially susceptible to the condition. The callus develops from pressure and friction to help protect the skin from damage.
Providing padded bedding often is very helpful in preventing the callus from worsening. All larger breeds should be given a thicker bed of some kind to prevent excessive pressure on the bony parts of the body. Foam rubber mats, air mattresses, and straw shavings all will provide support to protect against callus formation. Some people even have used elbow pads or foam placed around the dog's elbows to prevent calluses.
A callus is rarely a medical problem, unless it becomes ulcerated and subsequently infected. A veterinarian will be able to verify if the callus is problematic.