| Barnes' Notes on the Bible An horse - The reference here is undoubtedly to the war-horse. See the notes at Psalm 20:7. Is a vain thing - literally, is a "lie." That is, he cannot be confided in. For safety - For securing safety in battle. He is liable to be stricken down, or to become wild and furious so as to be beyond the control of his rider; and however strong or fleet he may be, or however well he may be "broken," yet none of these things make it certain that the rider will be safe. God is the only being in whom perfect confidence can be reposed. Neither shall he deliver any by his great strength - Safety cannot be found in his mere "strength," however great that may be. These illustrations are all designed to lead the mind to the great idea that safety is to be found in God alone, Psalm 33:18-19.
Clarke's Commentary on the BibleA horse is a vain thing for safety - Even the horse, with all his fleetness, is no sure means of escape from danger: the lion or the tiger can overtake him or he may stumble, fall, and destroy his rider.
Gill's Exposition of the Entire BibleA horse is a vain thing for safety,.... Though it is prepared for the day of battle, and is a very warlike creature, and of great service in war, yet safety only is of the Lord, Proverbs 21:31; this is put for all kinds of military preparations which men are apt to trust in, but should not, for they are "a lie" (i), as the horse is here said to be; that is, deceives and disappoints when trusted to; in like manner the olive is said "to lie", Habakkuk 3:17; when hope of fruit from it is disappointed; so "fundus mendax" in Horace (k); neither shall he deliver any by his great strength; in the time of battle; either by fighting for him, or fleeing with him. (i) "mendacium", Pagninus, Montanus; "fallax", V. L. (k) Epod. l. 1. Ode 16. ver. 45. Carmin. l. 3. Ode 1. v. 30. "Spem mentita seges", ib. Epist. l. 1. ep. 7. v. 87.
Geneva Study BibleAn horse is a vain thing for safety: neither shall he deliver any by his great strength.
Wesley's Notes 33:17 An horse - Though he be strong and fit for battle, or for flight, if need requires. And so this is put for all warlike provisions. Vain things - Heb. a lie; because it promises that help and safety which it cannot give.
Jamieson-Fausset-Brown Bible Commentary17. On the war horse (compare Job 39:19-25). a vain thing-a lie, which deceives us.
Matthew Henry's Concise Commentary33:12-22 All the motions and operations of the souls of men, which no mortals know but themselves, God knows better than they do. Their hearts, as well as their times, are all in his hand; he formed the spirit of each man within him. All the powers of the creature depend upon him, and are of no account, of no avail at all, without him. If we make God's favour sure towards us, then we need not fear whatever is against us. We are to give to him the glory of his special grace. All human devices for the salvation of our souls are vain; but the Lord's watchful eye is over those whose conscientious fear of his name proceeds from a believing hope in his mercy. In difficulties they shall be helped; in dangers they shall not receive any real damage. Those that fear God and his wrath, must hope in God and his mercy; for there is no flying from him, but by flying to him. Let thy mercy, O Lord, be upon us; let us always have the comfort and benefit, not according to our merits, but according to the promise which thou hast in thy word given to us, and according to the faith thou hast by thy Spirit and grace wrought in us. | 
