Earlier quoted context omitted.
Even the worst possible environment...say, repeated concussions to the head via a hammer, will still result in a human scoring far above any non-human. I'm pretty sure you're wrong. I think research suggests that dolphins, parrots, bonobos and some other animals have tested higher than humans.
At standard IQ tests? Surely the creature doing a a standard IQ test needs at the very least opposable thumbs? There will of course be exceptions, but a brain damaged but somewhat functioning human will always score better than a non-human at human-designed intelligence tests. The argument isn't that a dolphin or bonobo isn't potentially smarter than a brain damaged human, but that by designing our tests for human us…
Intelligence between humans and animals can (or could) be compared, but--like when testing between human populations--you wouldn't use a test that is so heavily biased against one of the groups that they can't even attempt it.