It could possibly be a hernia yes, but they are more common in pups I think. If it is a hernia they can sometimes be relatively harmess unless your dog is in pain.
We had a Staffy in our family for almost 15 years. As he aged lots of lumps and bumps seemed to appear on him, none of which caused him any pain. They were not a danger to his health in any way, just old age I think!
I've came across all sorts of lumps and bumps on pets. My lab has 2, one in his armpit and one on his chest. When I touch them to see if they have increased in size he does not yelp or seem in pain so I've deemed them harmless and they have not affected his health.
Put your mind at ease for now if your dog isn't in any pain. If this lump appears to get any bigger over time, then is the time that I would start to get concerned.
It could be cancer. My jack russell mix has the same thing going on right now. You can try giving him predisone 2x a day to see if this will help to shrink it, otherwise with his age, as long as it is not bothering him, I would just keep an eye on it.
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