1.  leviathan(n = noun.person) - the largest or most massive thing of its kind; "it was a leviathan among redwoods"; "they were assigned the leviathan of textbooks"
is a kind of freak, lusus naturae, monster, monstrosity

2.  leviathan(n = noun.person) - monstrous sea creature symbolizing evil in the Old Testament;
is a kind of mythical creature, mythical monster

1.  An aquatic animal, described in the book of Job, ch. xli., and mentioned in other passages of Scripture. [1913 Webster]
" It is not certainly known what animal is intended, whether the crocodile, the whale, or some sort of serpent." [1913 Webster]

2.  The whale, or a great whale. Milton. [1913 Webster]

leviathan, n.
1 Bibl. a sea-monster.
2 anything very large or powerful, esp. a ship.
3 an autocratic monarch or State (in allusion to a book by Hobbes, 1651).

Etymology:
ME f. LL f. Heb. liwyatan

N  size, magnitude, dimension, bulk, volume, largeness, greatness, expanse, amplitude, mass, proportions, capacity, tonnage, tunnage, cordage, caliber, scantling, turgidity, corpulence, obesity, plumpness, embonpoint, corporation, flesh and blood, lustihood, hugeness, enormity, immensity, monstrosity, giant, Brobdingnagian, Antaeus, Goliath, Gog and Magog, Gargantua, monster, mammoth, Cyclops, cachalot, whale, porpoise, behemoth, leviathan, elephant, hippopotamus, colossus, tun, cord, lump, bulk, block, loaf, mass, swad, clod, nugget, bushel, thumper, whooper, spanker, strapper, Triton among the minnows, mountain, mound, heap, largest portion, full size, life size, large, big, great, considerable, bulky, voluminous, ample, massive, massy, capacious, comprehensive, spacious, mighty, towering, fine, magnificent, corpulent, stout, fat, obese, plump, squab, full, lusty, strapping, bouncing, portly, burly, well-fed, full-grown, corn fed, gram fed, stalwart, brawny, fleshy, goodly, in good case, in good condition, in condition, chopping, jolly, chub faced, chubby faced, lubberly, hulky, unwieldy, lumpish, gaunt, spanking, whacking, whopping, walloping, thumping, thundering, hulking, overgrown, puffy, huge, immense, enormous, mighty, vast, vasty, amplitudinous, stupendous, monster, monstrous, humongous, monumental, elephantine, jumbo, mammoth, gigantic, gigantean, giant, giant like, titanic, prodigious, colossal, Cyclopean, Brobdingnagian, Bunyanesque, Herculean, Gargantuan, infinite, large as life, plump as a dumpling, plump as a partridge, fat as a pig, fat as a quail, fat as butter, fat as brawn, fat as bacon, immeasurable, unfathomable, unplumbed, inconceivable, unimaginable, unheard-of, of cosmic proportions, of epic proportions, the mother of all, teh granddaddy of all.