The mysterious Hebrew words Behemoth and Leviathan both designate monsters or wild beasts of some unspecified kind. The Vulgate retains the Hebrew words untranslated, and from there they have passed into most English Bibles. Curiously, however, the Septuagint doesn’t use either of them, giving Greek words instead. Behemoth, which occurs only once in the Bible (Job 40:10), appears in the Septuagint as θηρίον (
therion), a common word that can mean a wild animal of any kind. Leviathan occurs six times, and in five of them it is translated as δράκων (
drakon), a dragon. In the one remaining verse (Job 3:8) it appears as κῆτος (
ketos), the same word that @Todd_Easton quoted here as the Greek word for “whale” in Daniel.
Ronald Knox says in a footnote to Job 40:10:
[1] Behemoth is usually identified with the hippopotamus, sometimes with the elephant; Leviathan with the crocodile, sometimes with the whale. But both may be allegorical representations of the hostile powers overcome by the Creator.
http://newadvent.com/bible/job040.htm