"You will tread on the lion and the adder, the young lion and the serpent you will trample under foot. Those who love me, I will deliver; I will protect those who know my name. When they call to me, I will answer them. I will be with them in trouble, I will rescue them and honor them. With long life I will satisfy them, and show them my salvation" (Psalm 91:13-16).
What is the metaphysical meaning of verse 13? What is the lion, the young lion, the adder (or "cobra" in King James), and the serpent? What is the difference between the adder and the serpent, and then the lion and the young lion? And how does it fit with verses 14-16?
Certainly it is a ringing affirmation of the Power of the Divine as absolute protection through any challenges life may offer. Metaphysically, we understand 'angels' to represent divine ideas, emanating from Divine Mind and carrying divine guidance, wisdom, and inspiration to our human consciousness. They won't break down our resistance and force their way in, these angels. We must make our consciousness open and receptive to these divine ideas, placing our trust in them rather than in our limited human perspective. When we do—when we choose to dwell in the place of the Most High within us—it is assured that we will be guided safely through all challenges.
It's important to note that there will still be challenges to face. The promise isn't that all challenges will melt away, but that by trusting in our spiritual oneness we will move through challenges easily and fearlessly. The images you quote are emblematic of the challenges that, while still present in our mortal lives, will no longer have power over us. I wouldn't get too concerned about literal meanings of specific words; remember, the psalms are creative writing, not philosophic treatises.