The speaker describes a kind of frantic effort to escape, working and sweating to get away from something. The title, "Freak show," sets the scene: the speaker seems to feel like an oddity, something to be stared at. The body described, "all foot and fin," is awkward and half-formed, part land creature and part sea creature, and the itching suggests a restless discomfort in one's own skin.
The turn comes in the last two lines. What the speaker is running from isn't an outside threat but themselves: "I am who I fear seeing / in my closet at night." The closet is a classic hiding place for monsters, but here the monster is the speaker. The poem seems to be about self-disgust or shame, the fear of facing the part of yourself you consider strange or unacceptable. The mood is anxious, uneasy and a little darkly funny, since the poem is so short and the realization lands so suddenly.