This is a mare whose odd white streaks are probably the result of a somatic mutation. Somatic mutations are the result of a gene that is accidentally switched "off," often resulting in odd patches of color on an otherwise ordinary coat.
This dog has the black spot phenomenon on his face. The black spot phenomenon happens in golden and yellow Labrador retrievers when dog experiences a somatic mutation that turns the e/e to E/e. e/e is the genotype that produces the yellow to red coat color in golden retrievers and yellow Labs. If they had the E/e or E/e genotype, they would either be black dogs or liver/chocolate dogs. It’s not a really a birthmark. It’s just a somatic mutation.