IIRC animals gender is based on the art and scripture. Some animals, like the lions mentioned above, are obviously male/female. Other times the art doesn’t give a clue, like Fire Foxes. Sometimes the scripture refers to the animal in the masculine or feminine, like Leviathan.
The methods used to determine gender aren’t just limited to animals, it’s used for all characters (Faithful Servant promo anyone?). Animals are just less clear than most people.
In he case of Fire Foxes, and many other animals, we cannot tell using the above methods. Since they can be both male and female they are treated as genderless.
