I kept Biscuit
Im playing my 'evil' playthrough even though there's no actual moral repercussions and I decided that I would do the very opposite thing of what I would actually do.
So now I have Biscuit in my vivarium because it was the most evil option and now I feel bad 🥲 I am playing on hard mode so that my character gets knocked around for it.
Fun side note : hes gryffindor, not Slytherin