dog [noun] a domestic, meat-eating animal related to the wolf and fox.

you're a dog.

[ʃjɛ̃ ] masculine noun. 1. (= animal) dog. « attention, chien méchant » “beware of the dog”

It's masculine, because it just is. Many animals have a masculine and feminine version. So we have “le chien” and “la chienne.” They are easy to distinguish (apart from the article) because the feminine version double the n and adds an e. That's a typical feminine ending.