He saith among the trumpets, Ha, ha - That is," When the trumpet sounds, his voice is heard "as if" he said, Aha - or said that he heard the sound calling him to the battle." The reference is to the impatient neighing of the war horse about to rush into the conflict.
And he smelleth the battle afar off - That is, he snuffs, as it were, for the slaughter. The reference is to the effect of an approaching army upon a spirited war-horse, as if he perceived the approach by the sense of smelling, and longed to be in the midst of the battle.
The thunder of the captains - literally, "the war-cry of the princes." The reference is to the loud voices of the leaders of the army commanding the hosts under them. In regard to the whole of this magnificent description of the war-horse, the reader may consult Bochart, "Hieroz." P. i. L. ii. c. viii., where the phrases used are considered and illustrated at length. The leading idea. here is, that the war-horse evinced the wisdom and the power of God. His majesty, energy, strength, impatience for the battle, and spirit, were proofs of the greatness of Him who had made him, and might be appealed to as illustrating His perfections. Much as people admire the noble horse, and much as they take pains to train him for the turf or for battle, yet how seldom do they refer to it as illustrating the power and greatness of the Creator; and, it may be added, how seldom do they use the horse as if he were one of the grand and noble works of God!

He saith among the trumpets, Ha, ha - The original is peculiarly emphatical: האח Heach! a strong, partly nasal, partly guttural sound, exactly resembling the first note which the horse emits in neighing. The strong, guttural sounds in this hemistich are exceedingly expressive: האח ומרחוק יריח מלחמה Heach! umerachok yariach milchamah; "Heach, for from afar he scenteth the battle."
The reader will perceive that Mr. Good has given a very different meaning to Job 39:20 from that in the present text, Canst thou make him afraid as a grasshopper? by translating the Hebrew thus: -
"Hast thou given him to launch forth as an arrow?"
The word ארבה arbeh, which we translate locust or grasshopper, and which he derives from רבה rabah, the א aleph being merely formative, he says, "may as well mean an arrow as it does in Job 16:13, רביו rabbaiv, 'His arrows fly around me.'" The verb רעש raash in the word התועישנו hatharishennu, "Canst thou make him afraid?' he contends, "signifies to tremble, quiver, rush, launch, dart forth; and, taken in this sense, it seems to unite the two ideas of rapidity and coruscation." This is the principal alteration which this learned man has made in the text.
I shall conclude on this subject by giving Coverdale's translation: Hast thou geven the horse his strength, or lerned him how to bow down his neck with feare; that he letteth himself be dryven forth like a greshopper, where as the stout neyenge that he maketh is fearfull? He breaketh the grounde with the hoffes of his fete chearfully in his strength, and runneth to mete the harnest men. He layeth aside all feare, his stomach is not abated, neither starteth he aback for eny swerde. Though the qyvers rattle upon him, though the speare and shilde glistre: yet russheth he in fearsley, and beateth upon the grounde. He feareth not the noise of the trompettes, but as soone as he heareth the shawmes blowe, Tush (sayeth he) for he smelleth the batell afarre of, the noyse, the captaynes, and the shoutinge. This is wonderfully nervous, and at the same time accurate.

He saith among the trumpets, ha, ha,.... As pleased with the sound of them, rejoicing thereat, and which he signifies by neighing;
and he smelleth the battle afar off; which respects not so much the distance of place as of time; he perceives beforehand that it is near, by the preparations making for it, and particularly by what follows; so Pliny (b) says of horses, they presage a fight. The thunder of the captains, and the shouting; they understand an engagement is just about to start by the loud and thundering voice of the captains, exhorting and spiralling up their men, and giving them the word of command; and by the clamorous shout of the soldiers echoing to the speech of their captains; and which are given forth upon an onset, both to animate one another, and intimidate the enemy. Bootius (c) observes, that Virgil (d) and Oppianus (e) say most of the same things in praise of the horse which are here said, and seem to have taken them from hence; and some (f) give the horse the preference to the lion, which, when it departs from a fight, never returns, whereas the horse will. This is an emblem both of good men, Zac 10:3; and of bad men, Jeremiah 8:6.
(b) Nat. Hist. l. 8. c. 42. (c) Animadvers. Sacr. l. 3. c. 6. s. 1. (d) Georgic. l. 3. (e) Cyneget. l. 1. (f) Horus Aegypt. apud Steeb. Coelum Sephirot. Hebrews. c. 6. s. 1. p. 106.

saith--poetically applied to his mettlesome neighing, whereby he shows his love of the battle.
smelleth--snuffeth; discerneth (Isaiah 11:3, Margin).
thunder--thundering voice.

Ha, ha - An expression of joy and alacrity declared by his proud neighings. Thunder - The loud and joyful clamour begun by the commanders, and followed by the soldiers when they are ready to join battle.