Charles Ellicott Commentary Psalms 91:13

"Thou shalt tread upon the lion and adder: The young lion and the serpent shalt thou trample under foot." — Psalms 91:13 (ASV)

Lion ... adder ... young lion. —These are used no doubt, emblematically for the various obstacles, difficulties, and danger which threatens life. (For “adder,” see Note, Psalms 58:4; “dragon,”Psalms 74:13.)