I'm not an expert on the breed, but many large dogs develop fat pockets on the underside, usually more forward, like chest in my limited experience.
This is more common in older dogs, and for big dogs that is much earlier, maybe 7 for a Newfoundlander.
Get it check, and removed with a biopsy taken is my advice.
Big dog, Newfoundlander. That lump you discovered might be just Lipoma, a fat deposit. Lipoma can develop into several more lumps on your dogs body.
There are various types of lipoma, the best way to diagnose the type is by visiting your vet and let him run tests on your pooch lump.