Kinda looks like a pneumothorax. I only know of this in humans but I'm sure it could happen to an animal. But to explain this is non medical terms lol it's when air escapes from the lungs. But the air usually stays in the thoracic cavity. To be under the skin it's subcutaneous emphysema . Which could be caused by a small hole in the trachea that slowly leaked air. But I don't know for sure i just know how it happens in people. Keep us updated . That's just crazy!