Cat

Fogs and cats are one and the same in this poem. They're fused into one solid metaphor right from the beginning. First the fog approaches "on little cat feet" and then sits on "silent haunches" before "mov[ing] on." So we see the fog moving in the same way a cat does. In moving like a cat, we also assume that the fog will behave like one too in the same sort of unpredictable, mysterious, maybe even dangerous way. So we leave the poem with the possibility of the outcome of the fog perhaps turning out to be a bit different next time. That "mov[ing] on" part suggests that we're not sure where it will go or what it will do.

• Line 2: The fog is on "little cat feet" and we know those little paws can move rather quickly if they want to and can jump pretty far too. There are lots of questions in those little feet. 
• Lines 3-4: By the second half of the poem, the fog and cat are one. Wherever the fog goes, we see it as a cat. So here it's sitting and looking in that cool catlike way. Maybe kitty is impressed, maybe not, but we'll never know what kitty is thinking.
• Line 5: Those "silent haunches" carry lots of connotation in only two words. The "silent" part seems to evoke our sense of question and uncertainty, while the "haunches" allude to the potential of the cat-fog's power and speed. 
• Line 6: But by the very end it "moves on," so we're not so anxious anymore. Kitty isn't interested in pouncing on our heads today. That's not to say she won't be tomorrow though…