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It's just a birthmark. And just like freckles and moles, they can show up after birth. It doesn't mean the dog isn't purebred, and it's pretty common in Goldens.
HOWEVER, that said, when a new spot shows up on a dog's tongue, very very rarely, it can be an oral melanoma. Melanomas are often raised at least slightly, and an experienced vet can tell you in about two seconds that a spot is not a melanoma. It would be extraordinarily rare (and I've never heard of it) for a melanoma to completely mimic a spot of pigment.
If your dog gets a new one, keep an eye on it. If it continues to change shape, get larger, or becomes raised, then I'd worry. But like I said, I've never heard of a dog who had a melanoma that mimicked a pigment spot so completely that a vet missed it.
Oral melanomas are dangerous, so keep an eye on new spots, just like you'd keep an eye on human moles that change shape. Don't panic, though, 99.99% of the time, tongue spots are treat spots.