If your vet doesn't know, then it obviously isn't common, or else your vet isn't as knowledgeable as he should be. When a vet doesn't know about something like this, they should at least research it, or send you to a vet who does know. You need to take your dog to another vet for second opinion. It could be a tumor (benign or malignant), or it could be a cyst, among other things. But don't risk your dogs life by waiting.