Albert Barnes 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)

Thou shalt tread upon the lion and adder — You will be safe among dangers, as if the rage of the lion were restrained, and he became like a lamb, and as if the poisonous tooth of the serpent were extracted. .

The word used here to denote the “lion” is a poetic term, not used in prose. The word rendered “adder” is, in the margin, asp. The Hebrew word, פתן pethen, commonly means viper, asp, or adder. (See Job 20:14, note; Job 20:16, note; Isaiah 11:8). It may be applied to any venomous serpent.

The young lion — The “young” lion is mentioned as particularly fierce and violent. .

And the dragon ... — Hebrew, תנין tannı̂yn. (See Psalms 74:13, note; Job 7:12, note; Isaiah 27:1, note).

In Exodus 7:9-10 and Exodus 7:12, the word is rendered serpent (and serpents); in Genesis 1:21 and Job 7:12, whale (and whales); in Deuteronomy 32:33, Nehemiah 2:13, Psalms 74:13, Psalms 148:7, Isaiah 27:1, Isaiah 51:9, and Jeremiah 51:34, as here, dragon (and dragons); and in Lamentations 4:3, sea monsters. The word does not occur elsewhere.

It would perhaps properly denote a sea monster, yet it may be applied to a serpent. Thus applied, it would denote a serpent of the largest and most dangerous kind; and the idea is, that he who trusted in God would be safe amidst the most fearful dangers, as if he should walk safely amidst venomous serpents.