Lumps and bumps can come from alot of things. No animals, in general, have random bumps on their head. Some breeds of dog have a "keel" that runs down the center of their head, but this is the natural shape of the cranium and is normal (see catahoula leopard dog, great dane, etc). Swelling on the head that makes it appear as though there are lumps can come from trauma, hydrocephalus, insect bites/stings, abscesses, tumors, etc.
See the two photos and compare them: The first is a golden, the second a great dane. As you can see, the golden has a completely smooth area atop his head, the great dane has a more "pointed" look. This is the "keel" I speak of. If the area on your dog's head looks different, you need to see your vet for determination on what it could be. Again, it can range from an insect bite to a tumor. Sometimes its minor, sometimes its major. I'd check with your veterinarian as soon as possible.
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