I think horses can be both, and I agree with the statement that they are neither - more of a partner.
However, there are horses who are raised to be worked, such as farm horses were before they were replaced by machinery.
Now I think they are viewed more as pets by many, rather than working animals.
Now, it may just be the rider in me, but I give my car (a tool) a pat when it's done something good (like rev up to pass another car at higher speeds)... or I'm just insane...
I can understand both points of view, but my horses were both show horses and pets - I have cried into their manes when I had a bad day, played with them in their paddocks, and hung out with them "just cause," but I have also won many championships on them as well.
... maybe that was because I wasn't allowed a dog when I was a kid... :P