I'd say everything has several opposites, each in a different aspect.

If we're following bizzaro world logic, the opposite of anything is the version that's stupid, or if you're stupid, then smart.

I imagine bizarro me is teaching physics in a prestigious university.

Humans (like most living things) like to put things in boxes to make navigating their environment simpler. Temperatures aren't really opposites because they're on a continuum, but we talk about hot and cold as opposites to bring order to environmental navigation. The same with presence and absence of light and bright/dark. So, extending this logic, the opposite of cat is the absence of cat, but that doesn't really make sense. If you cut a cat in half, are you halfway on a continuum from cat to the opposite of cat? If the cat is whole but has died, is it cat, opposite of cat, something else entirely? I have no idea, and no one else really does either. Brains are weird.