If people know the actual sex of the animal in question, commonly, but not limited to, pets, they tend to use the appropriate gender pronoun. As has been noted, cats of unknown sex tend to get the feminine pronoun and dogs of unknown sex get the masculine pronoun. For what it's worth, back when grammatical gender meant something in English, cat (catt) was a feminine noun and dog (docga) was masculine, as was hound (hund).