Jamieson, Fausset and Brown on Job 39:19

19. The allusion to "the horse" (Job 39:18), suggests the description of him. Arab poets delight in praising the horse; yet it is not mentioned in the possessions of Job (Job 1:3; 42:12). It seems to have been at the time chiefly used for war, rather than "domestic purposes."

thunder--poetically for, "he with arched neck inspires fear as thunder does." Translate, "majesty" [Umbreit]. Rather "the trembling, quivering mane," answering to the "vibrating wing" of the ostrich (see on Job 39:13) [Maurer]. "Mane" in Greek also is from a root meaning "fear." English Version is more sublime.