Behemoth was clearly the hippo (then as now a very formidable animal, hippos in Africa kill more people annually than any other animal).
Leviathan was a local version of the age-old dragon myth, which dates back to at least 5000 BC with the Sumerians of Mesopotamia, the right time and place for the writers of the Bible to have heard and believed it and incorporated it into their writings.
These may be the remains of a biblical monster:
