In fa-ʿanna lanā sirbun, ʿanna is a past-tense verb and sirbun its postponed subject: a herd suddenly appeared. In kaʾanna niʿājahu, niʿāja is the noun of kaʾanna, and the pronoun refers back to the herd.
ʿAdhārā dawārin provides the comparison, while fī mulāʾin mudhayyalin describes the women’s flowing garments.
Rather than portray these animals in a fierce hunting image, the verse first gives a serene human analogy. The next line emphasizes the herd’s movement and arrangement.