There is no suggestion that the ancient Hebrews believed in the fire-breathing dragons of medieval lore, but they did believe in supernatural monsters loosely described as 'chaos monsters'. These were mythological creatures that inhabit creation stories and had to be defeated in order to bring order to the world.
Behemoth literally means 'great beast' and God defeated Behemoth, who now eats grass. Leo G. Perdue (Wisdom in Revolt: Metaphorical Theology in the Book of Job, page 226) says that Behemoth is a creature that God made and that, in the theopoeic realm of myth and ritual, symbolises the power of chaos. If God can defeat such an awesome creature, how much more easily can he deal with the feeble challenge of a mortal Job.
Leviathan is the fire-breathing monster of Job chapter 41. Unlike the medieval dragons, he was a sea monster and had many heads that God broke (Psalm 74:-14). Leviathan is well known in Canaanite mythology, associated with names such as the seven-headed Litan.
Robert Sutherland (Putting God on Trial: The Biblical Book of Job) says the Behemoth is a Hebrew chaos monster similar to Rahab (Psalm 89:10, Isaiah 51:9), Yam and Leviathan. In fact, he believes that Behemoth is Leviathan, although he agrees that some scholars see otherwise.