An horse - The reference here is undoubtedly to the war-horse. See the notes at Psalms 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, Psalms 33:18-19.