This was a kinda cool list . . . although, yeah, the tone wasn't terribly Christian. (I wish they had included the big black thing from Daniel 7:7 - "After this I kept looking in the night visions, and behold, a fourth beast, dreadful and terrifying and extremely strong; and it had large iron teeth. It devoured and crushed and trampled down the remainder with its feet; and it was different from all the beasts that were before it, and it had ten horns.")
I tend to think that both the Behemoth and the Leviathan are creatures which we are zoologically unfamiliar with. Their descriptions really don't coincide with the known animals they are often associated with (hippo/whale/croc/etc). In a poetic sense they represent uncontrollable forces of the world, that only God has mastery over. The Leviathan in particular is, in Jewish literature, a terrifying, chaotic, near-demonic creature from the sea. In Old Testament texts, the sea is indeed seen as a place of choas and evil, as well as the abode of the dead (sheol). Again, poetically, Leviathan represents those things. As for what known animal I would be comfortable associating with them, I don't think there is any that fits the biblical description.
With the Nephilim, and this is my opinion which I have the utmost humility with, it seems to me that we are seeing a sort of sexual reproduction between fallen angels and man. I tend towards this viewpoint for the following reasons:
1. Nephilim literally means "fallen ones".
2. The coupling of the "sons of God" and the daughters of men result in these nephilim, who are described as men of renowned. A coupling of a non-godly and godly person does not result in a hero . . . just a normal baby.
3. The narrative links these couplings directly to the great flood. They are seen as terribly evil and are at least a part of the reason God flooded the earth. But ungodly people have married and will continue to marry godly people until the end of the age. It seems like destroying almost all of humanity because of such relationships would be a bit of an extreme measure.
Anyhow, those are some of my thoughts as I've pondered them over the years. I don't think anyone should chose to die on these doctrinal hills. They're really minor issues . . . although really fun to think about.