According to Catholic sources, what is the "Leviathan" in the Bible? According to Catholic sources1, what is the Leviathan in the Bible?

Is it a mythical creature or a particular animal that it can be identified with or some other creature?

1. E.g. Catholic Biblical commentary, Church Fathers, saints, etc. The best answer will reference these.



cf. Leviathan and Behemoth - JewishEncyclopedia.com.
From the Catholic Encyclopedia:


  LEVIATHAN. — The word Leviathan (Hebrew, líweyãthãn), which occurs six
  times in the Hebrew Bible, seems to have puzzled not a little all
  ancient translators. The D.V. has kept this name, Job 3:8; 40:20;
  Isaiah 27:1; it is rendered by dragon Psalm 73:14, and 103:26; The
  word leviathan means:
  
  
  (1) crocodile (Job 40:20 and Psalm 73:14);
  (2) a sea-monster (Psalm 103:26, Isaiah 27:1);
  (3) possibly the Draco constellation (Job 3:8).
  


In Haydock's commentary on Job 40, he says much the same thing; in Job 40 the reference is to the crocodile, but the reference could also be a whale or dragon. Thomas Aquinas says it is "some great fish" but could also be a reference to the devil. The devil reference is supported by Haydock's commentary on Job 3, in which he says:


  The fathers generally understand the devil to be thus designated.
  Septuagint, "he who is about to seize the great whale," (Haydock) or
  fish, which they also explain of the conflict of Satan with Jesus
  Christ. (Origen, &amp;c.)


Footnote to Job [40:25], Job, chapter 40 | USCCB (New American Bible) has: Leviathan: although identified by some scholars as the crocodile, it is more likely another chaos monster and the Jerusalem Bible Popular Edition footnote to Job 3:8 has: The dragon of primeval chaos; he might be roused by a curse against the present order.

Clearly, there is no consensus within or without Catholicism on what exactly the leviathan is. But they seem to agree it is a large aquatic or semi-aquatic beast or a chaos monster, and possibly a reference to the devil.