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Don José, you confirmed what I suspected.
I'm not certain, but I think that the term "pastor belga" as the name of a breed of dog is perceived by many people as a noun being used as an attribute of another noun (in this case, "perra"). From what I can find out, more often than not nouns used this way are NOT changed to agree in gender and number with the nouns that they modify, in the same manner as many terms for specific colors such as "malva" and "marrón".
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