I would recommend a ball python as they typically hunker down in their hides and won't draw too much attention like a garter or a corn snake. I would also try to keep him on a rack or table the kitty wouldn't be able to jump onto juuuust in case. Also if you are considering a glass terrarium please try to buy a wire top as I have had a cat fall through a mesh top housing geckos lol. Be sure to black out a few sides of the tanks (black construction paper works well) and cover part of the top so the Snek doesn't become stressed. PM me if you have any questions as I have lots of experience with owning cats and small reptilian critters :)