Sea monsters

Moby Dick, chapter 45: 

"In the sixth Christian century lived Procopius, a Christian magistrate of Constantinople, in the days when Justinian was Emperor and Belisarius general. As many know, he wrote the history of his own times, a work every way of uncommon value. By the best authorities, he has always been considered a most trustworthy and unexaggerating historian, except in some one or two particulars, not at all affecting the matter presently to be mentioned.
Now, in this history of his, Procopius mentions that, during the term of his prefecture at Constantinople, a great sea-monster was captured in the neighboring Propontis, or Sea of Marmora, after having destroyed vessels at intervals in those waters for a period of more than fifty years. A fact thus set down in substantial history cannot easily be gainsaid. Nor is there any reason it should be. Of what precise species this sea-monster was, is not mentioned. But as he destroyed ships, as well as for other reasons, he must have been a whale; and I am strongly inclined to think a sperm whale. And I will tell you why. For a long time I fancied that the sperm whale had been always unknown in the Mediterranean and the deep waters connecting with it. Even now I am certain that those seas are not, and perhaps never can be, in the present constitution of things, a place for his habitual gregarious resort. But further investigations have recently proved to me, that in modern times there have been isolated instances of the presence of the sperm whale in the Mediterranean. I am told, on good authority, that on the Barbary coast, a Commodore Davis of the British navy found the skeleton of a sperm whale. Now, as a vessel of war readily passes through the Dardanelles, hence a sperm whale could, by the same route, pass out of the Mediterranean into the Propontis.
In the Propontis, as far as I can learn, none of that peculiar substance called brit is to be found, the aliment of the right whale. But I have every reason to believe that the food of the sperm whale—squid or cuttle-fish—lurks at the bottom of that sea, because large creatures, but by no means the largest of that sort, have been found at its surface. If, then, you properly put these statements together, and reason upon them a bit, you will clearly perceive that, according to all human reasoning, Procopius's sea-monster, that for half a century stove the ships of a Roman Emperor, must in all probability have been a sperm whale." 

Chapter 59: 

"There seems some ground to imagine that the great Kraken of Bishop Pontoppodan may ultimately resolve itself into Squid. The manner in which the Bishop describes it, as alternately rising and sinking, with some other particulars he narrates, in all this the two correspond. But much abatement is necessary with respect to the incredible bulk he assigns it." 

Does the act of specifying these sea monsters make the mythological/ unknown creatures more ordinary, less mysterious and therefore less scary (because "[i]gnorance is the parent of fear"- chapter 3)? Or does it do the opposite- the frightening creature one'd like to dismiss as legendary and nonexistent is discomfortingly real? 
Throughout the novel, Ishmael constantly refers to the whale as leviathan. Are we meant to think that the act adds something extraordinary, magical and wondrous to the whale, which is already terrifying because of its monstrous size and strength, and our limited knowledge of the species? Or does it make the leviathan not only more "concrete" and specific but also more ordinary? Or does Ishmael/ Melville simply like to give the whale some religious/ symbolic meaning?