Here the speaker describes a state of lonely, restless thinking. The opening image is strange and memorable: the mind becomes "another ear," something built to listen, but with "no one to read to it." So the speaker's thoughts have no audience and nothing to take in. Then comes "an ache taken for a walk," which makes the pain feel like a pet or a restless child, something that has to be led around because it won't settle. The speaker seems to be going in circles with a feeling they can't put down.
The last two lines drop the metaphors and become plain: "It was dark / in my closet." A closet is a small, hidden place, and the title suggests a monster waiting there, though the poem never shows one. Judging by the imagery, the real fear may be isolation or the speaker's own troubled mind. The mood is quiet, hushed, and uneasy, with the sudden short ending leaving a chill.