Describe the dog that tries to share the barn with Billy to escape the rain.

The dog is obviously old and is crippled in its back legs. There is evidence of severe arthritis and the dog has an "ill-joined" gait so that he crabs sideways as he walks. The dog stops at the door, unable to see, and smells the air to figure out what Billy is. Then the dog slinks into the shelter and lies down on a pile of hay that he's obviously occupied before. Billy yells at the animal and throws a clod of mud at it, but the dog doesn't budge.