You have described to me a 10 month old male Shepherd mix that has a lump under his skin.
With any lump, testing would have to be done to be certain of what it is. This is the same as for a woman with a lump in a breast - no doctor is going to suggest a diagnosis based simply on finding a lump. Tests would be done to know what the lump is, and what needs to be done about it. In dogs, this often just means doing something called a "fine needle aspirate."
A fine needle aspirate (FNA) is easily done by inserting a small needle into the lump and aspirating (sucking out) some cells to be examined on a slide. This slide may be examined in-hospital by the vet, or sent out to a pathologist (a specialist in this area). The advantages of a FNA are that it is fast, minimally invasive, easy and the least expensive option . The disadvantage is that it is the least effective at making a diagnosis.
However, with your dog, there is a very good chance that not only would this give a diagnosis, but would also be an effective treatment. The reason that I say this is because what you are describing is probably either a seroma or a hematoma.
A seroma is a pocket of serum (the clear fluid left over when blood clots). A hematoma is a pocket of blood, basically a blood blister. It is otherwise just the same as a seroma. Serum is just the fluid left over when blood clots, so most hematomas become seromas as the blood inside clots - thus creating a little lump of blood clot and a ball of serum.
This could have occurred because there was oozing under the if the other dog pulled on the skin, or if your pup got his skin caught on a branch outside. Basically, it is a huge blister. Seromas are not unusual after rough-housing but this is an unusually big one.
Usually, with a seroma, I do drain it aseptically with a needle and syringe, and possibly give a course of antibiotics. Unfortunately, it often fills again, which indicates that the skin has not yet adhered to the underlying tissue. It is something that will eventually resolve on its own, though it may take several weeks to months for the body to absorb the serum such that the lump disappears. Alternatively, your vet could drain it again and then wrap a pressure bandage around your dog's chest so that the skin is held down to the underlying muscles so that it reattaches. This will hasten its disappearance, but really it would be for your comfort, not the dog's as he doesn't seem to be bothered by it!
So, in summary, this is likely a seroma or hematoma that will resolve on its own.
However, the only way to get a diagnosis would be to have your vet check your pup over, and aspirate the lump. If straw coloured or bloody fluid is removed by aspiration, this would not only give a diagnosis, but would treat the problem as it would remove the pocket of fluid. It might, however, fill up again.
If this has answered your question, please hit the "Accept" button and leave feedback. if you need more information, just click on reply and I'll still be here to provide it!
The above is given for information only. Although I am a licensed veterinarian, I cannot legally prescribe medicines or diagnose your pet's condition without performing a physical exam. If you have concerns about your pet I would strongly advise contacting your regular veterinarian.
Good luck with your boy!