Free Verse
This poem seems to be formally organized, but it's really all over the place. At first read, you see that the stanzas are all the same length (six lines apiece) and that there's definitely some rhyming going on. But, going back through the poem trying to figure out the rhyme scheme is like banging your head against the wall! In some stanzas, Lowell sticks to a perfectly predictable pattern. Look at the fourth stanza: ABCBCA. Perfectly neat. But none of the other stanzas follow that pattern exactly. They all have some rhyming in them. In fact, most of the time each end word (the last word in a line) has a rhyming match within the stanza, but when and where the match comes varies throughout the poem.
There's no predicting this man. The fake-out rhyme scattering that Lowell does in "Skunk Hour" mirrors the progress of the poem, and of our fragile-minded speaker.