"Behemoth, leviathan, Nephilim, giants, etc"
There are several things to consider when the Bible references animals. 1st. Where is it being referenced? The books in the later half of the Old Testament are all written in poetic form. Ideas and concepts are laid out in symbols.
This is the case with a behomoth and a leviathon and for that matter a unicorn. These animals are symbols. They were probably used because people had superstitions about all sorts of different kinds of creatures and these were being used in poetic form.
The behemoth spoken of in Job was clearly a representation of God's power. It is compared to an ox. The Hebrew word just means beast and that fits the description of oxen, cows, bison, etc.
A leviathan is a sea creature. Many times it is used in reference to sin and the chaos that sin can cause in someone's life. Once again it is used in all of the poetic writings in the Old Testament.
The unicorn is most likely just an ox. It is not a horse with a horn.
"There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown." Genesis 6:4 KJV
This is something that really bugs me. There are a whole lot of Christians that make a HUGE deal out of nothing with this verse.
Nephilim is the Hebrew word for giant. In the Hebrew text is where you find the word nephilim but in the KJV it is "There were giants...". But who does it say they were? It says, "when the sons of God came in unto the daughters of men". They "became mighty men" and "men of renown". There are a whole lot of Christians who read "sons of God" and coe up with some wild fantasy about gods who were fallen from heaven or some such nonsense. Who are the "sons of God"? Every male who has ever been born on this planet. Who are the "daughters of men"? Every female who has ever been born on this planet. The verse could have read "when the sons of men came in unto the daughters of God..." and it would mean the exact same thing. Basically this verse says that men and women had sex and produced a generation of mighty people. That's it. That's all it ever meant. The nephilim are not some mystical race of demigods. They were powerful and corrupt men. They were called giants because of how much power and control they had.