The Bible mentions several interesting creatures, not least among them Behemoth and Leviathan. The first monsters mentioned in Scripture are actually in Genesis 1. "And God created the great Tanninim [the standard Hebrew lexicon says, "serpent, dragon, sea monster"]" (Gen 1:21). Usually English translators wimp out with "the great sea creatures" (ESV) or "the great creatures of the sea" (NIV), but the NASB gets it right with "the great sea monsters." The 'im ending is plural. We know that people in all places throughout the world have believed in dragons and similar great monsters. The Ancient Near East was no different. These Tanninim were monstrous serpent-dragons of the sea. The difference between the creation account in Genesis 1 and other ANE accounts is that God created the Tanninim. In the myths, these kinds of monsters were rebels against the gods. But in Genesis 1, they are creatures of God. [For example, so says Waltke, Genesis, 63 (same page as later references)]
These myths were also coopted by the Biblical authors in Isaiah and Jeremiah and in Psalms and Job.
Isaiah 27:1 says, "In that day the LORD with his hard and great and strong sword will punish Leviathan the fleeing serpent, Leviathan the twisting serpent, and he will slay the dragon that is in the sea." Leviathan ("coiled one" according to Waltke) is one of these Tanninim (the Hebrew for "dragon" is Tannin). That day is The Day of the LORD. That is, on judgment day Leviathan will be slain. Leviathan becomes a reference to Satan. The Book of Revelation in the New Testament will pick this theme up saying, "And the great dragon was thrown down, that ancient serpent, who is called the devil and Satan, the deceiver of the whole world--he was thrown down to the earth, and his angels were thrown down with him" (Rev 12:9). "And he seized the dragon, that ancient serpent, who is the devil and Satan, and bound him for a thousand years" (Rev 20:2). Satan is no myth, but he is described with this language from ANE myths, which is not really surprising considering that sometimes he is called Beelzebul (some English translations Beelzebub) -- a name first ascribed to one of the false gods (one of the Baals) in the ANE pantheon. The false gods and the mythical creatures of chaos become names for Satan.
Another ANE Tannin was Rahab ("arrogant one" according to Waltke). Isaiah 51:9-10 says, "Awake, awake, put on strength, O arm of the LORD; awake, as in days of old, the generations of long ago. Was it not you who cut Rahab in pieces, who pierced the dragon? Was it not you who dried up the sea, the waters of the great deep, who made the depths of the sea a way for the redeemed to pass over?" Again, the Hebrew for "dragon" is Tannin. History repeats. At the crossing of the Sea of Reeds (commonly called the Red Sea in English Bibles), Rahab was pierced. At the New Exodus, Leviathan is pierced. Rahab was a poetic name for Egypt. It is not a stretch to think of Egypt as a serpent given the Pharaoh's headdress. And moreover, each of these nations worshipped demonic false gods and so the defeat of a nation was also a defeat of those gods -- and thus a defeat of the dragons that symbolized those gods. The final defeat foreshadowed in the curse on the land-serpent Satan, "he shall bruise your head" (Gen 3:15).
The prophet Jeremiah mentioned the Tanninim by way of analogy. He said, "Nebuchadnezzar the king of Babylon has devoured me; he has crushed me; he has made me an empty vessel; he has swallowed me like a monster; he has filled his stomach with my delicacies; he has rinsed me out" (Jer 51:34). Here the "monster" is a Tannin -- a dragon-serpent sea monster. This is similar to the Isaianic discussion about judgment day being a swallowing of the swallowing serpent.
Among the first two books of the Writings, these themes are even more prevalent. In Psalm 73 the psalmist writes, "You divided the sea by your might; you broke the heads of the sea monsters (Hebrew Tanninim) on the waters. You crushed the heads of Leviathan; you gave him as food for the creatures of the wilderness" (Psalm 73:13-14). Here the sea monsters are called Leviathan. It is important that this is a type of crushing the head of the great serpent (Gen 3:15). The Psalmist coopts this mythical language but in a different way than the rest of Scripture since he does not refer to this sea monster as Rahab but Leviathan. The Psalmist in Psalm 104 tells the story of creation in a similar way to Genesis. Psalm 104:26 says, "There go the ships, and Leviathan, which you formed to play in it [the sea]." As Waltke says in a footnote, this "reduces the Leviathan to a duck in God's bathtub." Waltke has it exactly right. The Tanninim, including Leviathan and Rahab, are but creatures of God that serve Him and have been/will be judged for their rebellion.
This brings us to Job. Job is an interesting book if for no other reason than the heavenly council vision at the start where Satan is a participant. But it brings up these ANE mythical creatures too. Job 3:8 says, "Let those curse it who curse the day, who are ready to rouse up Leviathan." Perhaps the eschatology of Job includes the loosing of Leviathan right before the final judgment (like Rev 20:3). But whatever the text means, Job is cursing the day of his birth and comparing it to evil eschatological events. Job mentions sea monsters in general, "Am I the sea, or a sea monster [Hebrew is Tannin], that you set a guard over me?" (Job 7:12). Job also refers to the death of the Tannin called Rahab in the same way as the prophets above, "By his power he stilled the sea; by his understanding he shattered Rahab. By his wind the heavens were made fair; his hand pierced the fleeing serpent" (Job 26:12). Here the word serpent is the same as Gen 3:1.
And just after mentioning Behemoth, Leviathan comes up again in Job 41:1-2, "Can you draw out Leviathan with a fishhook or press down his tongue with a cord? Can you put a rope in his nose or pierce his jaw with a hook?" And God continues this way. God can catch the 'mighty' Leviathan like a fish. The point is to emphasize God's strength and Leviathan's impotence and thus to challenge Job for confronting God.
This now brings us to the identity of Behemoth. The word is also a Hebrew transliteration. The idea is of a great wild animal of some kind. Most translations put in the footnote a hippopotamus or elephant based on the description, but the Hebrew word is a general one for an animal sometimes refering to cattle or other livestock. The Septuagint translates the word with a general term for a wild animal. But the point of the text is that both the sea monsters and the beasts on land are creatures of God that depend upon Him. God says, "Behold, Behemoth, which I made as I made you..." (Job 40:15). The book likes piercings: "Can one take him by his eyes, or pierce his nose with a snare?" (Job 40:24). This mighty beast depends on God for his food, shelter, and strength. Again, the point is to emphasize God's strength and Behemoth's impotence and thus to challenge Job for confronting God. Both of these creatures are not impotent compared to Job but they are compared to God.
And thus back to Satan. No matter whether he is seen as a serpent-dragon of the sea or a land-serpent, he is a creature of God and under His control. Satan is impotent compared to God. More than that, Satan is powerless except as God gives him authority. This is the teaching of Scripture throughout.
On a lighter note, Is Nessie, the Loch Ness Monster, one of the Tannin? She bears a striking resemblance to the plesiosaurus in the hoaxes. I cannot help but note a lot of similarities to the myths in the ANE in the way some have described this "sea serpent" or "dragon" before they settled on calling it the Loch Ness Monster according to the wikipedia article.
[edited 1/19/2012 for one letter typo]