Well, what *I* understand of Leviathan is this: The word is a collective, and so used for many things. Although each thing is specifically descriped. Serpent, Pharoah, etc. Similarly, we might use the word ‘monster’ to describe a bad person, an unknown shadow in a closet, or the size of something.
The best description is in Job: I cannot be silent about his limbs, his power, and his graceful proportions. So, it’s got limbs, whales don’t have limbs. Who can strip off his outer covering? Who can penetrate his double layer of armor? Thick skinned. Who can open his jaws, surrounded by those terrifying teeth? I dunno about you, but I’ve never even SEEN rhino teeth. [His] pride is in [his] rows of scales, closely sealed together. Elephants with scales?? I don’t think so. His snorting flashes with light, while his eyes are like the rays of dawn. Flaming torches shoot from his mouth; fiery sparks fly out! Smoke billows from his nostrils as from a boiling pot or [burning] reeds. His breath sets coals ablaze, and flames pour out of his mouth. I don’t know any animals that even closely resemble this.