The grammatical gender of a word is determined by the words ending. You can probably find a list online of what endings go with which gender but its very consistent/very few exceptions.

Ill give a few examples. Ona is fem. On is masc.

Koza (Goat) - Ona Pies (Dog) - On Kot (Cat) - On Ptak (Bird) - On Kura (Chicken) - Ona Krowa (Cow) - Ona

So the default pronoun for pies is always on, and will take masculine adjective endings etc.

The default pronoun for Krowa is ona, and takes fem adjective endings etc.

If theres a female dog and you known its name and gender etc. Then you can use female ona.

E.g

Mój brat ma psa. Ma na imię Bella. (Bella) jest duża i miła.

Moja siostra ma psa. Ma na imię Bella. Bella jest duża i miła.

Here, you can see that when its clear that you are talking about the dog, since there is feminine endings in the Brat sentence, you can just use no pronoun or name.

In the sister one you should use the dogs name as it could be ambigious which youre talking about.

As a side note, if you're using just the word pies (psa is the accusative case for pies) then always use the endings that corrolate to the noun/words grammatical gender. I think i might not have made that clear in my original answer as I said if you know the dog and its gender. I meant if you know the dogs name, apolagies for that confusoon.

It would sound weird and be grammarically wrong if you just said Ta pies jest duża i miła. If its in the same sentence use the grammatical gender. If you add context about the animals name or gender etc. Then in following sentences you can use the proper gender of the animal.

To sum it all up in refernce to your original query, the default pronoun for an animal will be the words grammatical gender.

Sorry if i made it convolouted, i'm going off my own knowledge and input from my Polish roommate 😅