The way line breaks are used in Marianne Moore's "The Pangolin" is quite distinctive, and a hallmark of Moore's very strong personal voice and style. For Moore, line breaks have several functions relating to the way a poem sounds and the way we read.
The first function of line breaks is that Moore tends to create lines using syllable counts as the basic sonic patters. Although these syllabic counts don't normally form an absolutely regular stanzaic form, you can observe patterns, especially in the way she intersperses longer with shorter lines.
The main function of the line breaks is to indicate places where you should pause in reading the poem aloud. Another part of her reasoning behind line breaks is to resist the naturally iambic pattern of English, and to create genuinely irregular rhythms. Finally, the odd breaks running against the syntactic flow of the poem focus the reader's attention and disrupt our ordinary automatic habits as readers, making us slow down and pay attention to each word.