Latin Lexicons, big and small, seemed to have different opinions on the gender for such nouns as mus. OLD, Gaffiot, Langenscheidt, Crooks and Schem, etc., only give one gender, which is masculine, as many introductory textbooks did. Lewis and Short, Cassell, Leverett, Smith, White and Riddle, etc., on the other hand, labeled it common or m & f. Some medieval Latin text used it as feminine.
Question is: what’s the guiding principle on gender, above and beyond lexicon authorities?