Quick answer: Most lumps on dogs are benign lipomas, but any new lump should be checked by a vet — mast cell tumors can look identical to benign lumps and require prompt action.
Lipomas are soft, movable fatty deposits just under the skin. Very common in middle-aged and older dogs, especially Labs. They feel soft and doughy, move freely when pressed, and grow slowly over months. They're almost always benign but should be confirmed by a vet — don't assume.
Mast cell tumors are the most common malignant skin tumor in dogs and can look exactly like a benign lump. They may change in size (appearing to shrink or enlarge quickly) and can be itchy. Any new lump on a dog — regardless of how harmless it looks — should be assessed.
See a vet promptly if the lump grows rapidly (days to weeks), is hard and fixed to underlying tissue, is ulcerated or bleeding, or if your dog seems unwell alongside the lump. A fine needle aspirate (FNA) is a quick, minimally invasive procedure that provides an initial assessment within minutes.
Most bumps on dogs are benign, but don't assume. Any new lump deserves a vet check — FNA is fast, inexpensive, and provides real information rather than guessing.