Verified answer

it will go away, that's just some kind of rash for the abnormal circulation of blood due to hypertension of something that happened which your dog's body was unable to cope up with.

It sounds like an abscess from trauma. It will not go away on its own. It needs to be lanced and drained by a vet, especially because it is a pretty big abscess.

But it is not life threatening. Keep the dog quiet and calm and get him to a vet as soon as you can, he can probably wait until after Christmas. Just make sure your two dogs don't wrestle because if the abscess bursts on its own, not only is it extremely painful, it has more of a risk of getting infected.