A worn-out white horse stands alone among the sagebrush on a cold day. The speaker watches him and wonders what is going through his mind. At first the answer seems simple: he must be thinking only of the cold and his tiredness. Then the poem asks a gentler question. If the horse were warm in a stable, belly full of straw, what would he think about then?
The last three short stanzas turn the poem with a joke. The horse would think of young mares, and the speaker puts it bluntly: a "tempest" stirring in his "itching balls." The shift from weary pity to crude comedy is sudden, and it is the heart of the poem.
The speaker seems affectionate and a little amused, not mocking. The mood moves from bleak and quiet to wry and earthy. The title, "Who Knows," admits we can't read an animal's mind, though the poem hints that desire outlasts age and comfort sets it free.