The poem follows a figure through one morning of small, slightly wild behavior. He shouts at the fields, paces up and down, smashes someone's windows, and then simply goes home. Nothing is explained, and that blankness is part of the effect: we see the actions but not the reasons.
The brackets are the most interesting part. Each one gives the figure a new name: "the crooked fellow," "the churchyard cock," "my dog-brother." They sound like the speaker trying on labels, half teasing, half affectionate. The last one, "my dog-brother," is the closest the poem comes to admitting a bond, and the title, "Alter ego," suggests this restless creature is a version of the speaker himself.
The mood is playful but a little uneasy. There's a sense of pent-up energy let loose, then dropped as easily as it began. The speaker seems to watch with amused recognition, perhaps even some envy of such unashamed noise.