It sounds to me like he is describing one of the long neck dinosaurs.
If you read the next part of Job, God describes another unknown animal - could it be another dinosaur? Many say a crocodile, but you read it and see if the crocodile is as fierce as Leviathan (by the way in Psalm 104:26 "There the ships sail about; There is that Leviathan Which You have made to play there." This is another description of leviathan "playing" with the ships in the sea).
We don't know for sure if these are dinosaurs the Bible is describing because the names are unknown. But we do know that everything that has ever lived on the earth was created by God during the first week of Creation. God says in Exodus 20:11 "For in six days the Lord made the heavens and the earth, the sea, and all that is in them, and rested the seventh day. . . "
