mouse

Cat is not the opposite of mouse. They have too many things in common. They're both mammals. They've both got four legs, two eyes, and a tail. Both cats and mice are commonly found in close proximity to humans. Cat teeth are designed to cut meat (mouse meat, for example) and mouse teeth are designed to gnaw through things, but nevertheless both kinds of teeth are clearly teeth.

So cat can't be the very opposite of mouse. I suggest perhaps »steel mill« or »the seas of Europa« as being the true opposite of mouse.