"Το κουτάβι δεν είναι μία γάτα."

Translation: The puppy is not a cat.

[_Dimitris_]

Το κουτάβι δεν είναι γάτα is more natural.

[Stergi3]

αγγλισμός. Anclisised phrase

[SzechuanSage]

Neither my parents nor my in-laws have ever used the word κουτάβι for "puppy". My mother said the word was used as an insult when someone did something dumb.

[troll1995]

I have never heard of such usage. That's probably κουτός (which means silly), not κουτάβι.
Do your parents live in Greece?