28 What then, when the sun went down, when our Lord suffered? There was a sort of darkness with the Apostles, hope failed, in those to whom He at first seemed great, and the Redeemer of all men. How so? “Thou made darkness, and it became night; wherein all the beasts of the forest shall move”....Here the beasts of the forest are used in different ways: for these things are always understood in varying senses; as our Lord Himself is at one time termed a lion, at another a lamb.
What is so different as a lion and a lamb? But what sort of lamb? One that could overcome the wolf, overcome the lion. He is the Rock, He the Shepherd, He the Gate. The Shepherd enters by the gate: and He says, “I am the good Shepherd:” and, “I am the Door of the Sheep.”...Learn thus to understand, when these things are spoken figuratively; lest perchance when you have read that the Rock signifies Christ, ye may understand it to mean Him in every passage. In one place it means one thing, another in another, just as we can only understand the meaning of a letter by seeing its position. “The lion's cubs roaring after their prey, do seek their meat from God”.
Justly then our Lord, when near unto His going down, the very Sun of Righteousness recognising His going down, said to His disciples, as if darkness being about to come, the lion would roam about to seek whom he might devour, that that lion could devour no man, unless with leave: “Simon,” said He, “this night Satan has desired to have you, that he may sift you as wheat. But I have prayed for you, that your faith fail not.” When Peter thrice denied, was he not already between the lion's teeth?...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)