Jamieson, Fausset and Brown on Isaiah 34:7

7. unicorns--Hebrew, reem: conveying the idea of loftiness, power, and pre-eminence (see on Job 39:9), in the Bible. At one time the image in the term answers to a reality in nature; at another it symbolizes an abstraction. The rhinoceros was the original type. The Arab rim is two-horned: it was the oryx (the leucoryx, antelope, bold and pugnacious); but when accident or artifice deprived it of one horn, the notion of the unicorn arose. Here is meant the portion of the Edomites which was strong and warlike.

come down--rather, "fall down," slain [Lowth].

with them--with the "lambs and goats," the less powerful Edomites (Is 34:6).

bullocks ... bulls--the young and old Edomites: all classes.

dust--ground.