The Psalms also speak of Leviathan. Psalm 74:14 says, "You crushed the heads of Leviathan; you gave him as food for the creatures of the wilderness" (ESV). A verse which seems to parallel this one is Genesis 3:15 which says, "And I will put enmity between you and the woman, and between your offspring and hers; he will crush your head, and you will strike his heel" (NIV).
Revelation 12 talks about dragon coming from the sea.
It ultimately doesn't really matter which it is. What is important to note here is this: Satan is NOT a mythical creature. He is VERY really and must not be forgotten or taken lightly.
However, despite all the power that he may have, that simple fact remains that the living God will, in the end completely destroy him, and along with him, all evil. The LORD God will triumph. Evil will one day be destroyed. And for that I am thrilled.