So you might think we're nuts, but we think this poem sounds like a cat's feet as it moves across a room. Just listen to the way these lines stop and start. The first line picks up speed, patters along gracefully, and then comes to a stop. Imagine the cat running across a table, stopping, and then starting up again. Sometimes even in the middle of a line the poem will pause, before heading off in a new direction, like in line 3: "If snow be white, why then her breasts be dun." We get a quick little dash through the white snow bit, then a pause at the comma, and we take off running again.
We thought about horse's hooves, galloping and then stopping, but do you hear how soft and elegant these lines are? Even when the image the speaker presents is gross or weird, there's a lightness to the sound. It manages to both stop and start and to flow along beautifully, just like an almost-silent, watchful cat dashing from place to place.