That looks like a small rat!

My ma had mice - i almost had a heart attack! It popped up on the back of the couch across the living room from me late one night and LOOKED at me. My poor ma was sleeping and i was about busting out of my skin. The thing was STINKY!

And he was a dirty gray. He didnt have a nice white belly like your buddy there. I wonder if one of your neighbors lost a pet or a school project or their snakes snack?

Your cat looks pretty happy to have a playmate!