This lump could be all kind of things. It could be a large cyst. It could be a fatty tumor (Lipoma, which is benign) -or it could be something more serious.
The only way to be certain is to take your dog to the vet and have him look at it. You might be advised to have a needle-biopsy done. That is fairly quick, and needs no anesthetic. A small sample of the tumor is taken with a syringe, and sent for analysis. Sometimes the results come almost immediately, as when the vet takes the sample and puts it on a slide, it can sometimes be obvious whether what it contains is just fatty substance. But most times there is a short wait for results, of a few days perhaps.
It is definitely worth doing this. If by chance it was a cancerous tumor, it would have to be removed as quickly as possible.
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