There's a few problems with identifying the behemoth with a large dinosaur. If one reads Job 40:17 carefully, it is apparent that it doesn't claim that a behemoth's tail is as big as a cedar. Instead, it says the behemoth "moveth his tail like a cedar" (KJV) or "His tail sways like a cedar" (NIV). Now, how does a cedar tree move? Here's a hint: it doesn't bend about like a lizard's tail (the NASB's humorous translation notwithstanding). It's stiff, straight (at least the cedars referenced in the Bible usually are), and only swaying a bit in the wind.
As for the size of the behemoth, that is hinted at by Job 40:21-22: "Under the lotus plants he lies, hidden among the reeds in the marsh. The lotuses conceal him in their shadow; the poplars by the stream surround him" (NIV). So, while this is a large creature that spends a lot of time in and by water, it can be hidden by reeds and surrounded by willows (KJV) or poplars (NIV). In order to reconcile its status as a large animal with being covered by reeds, it seems as though this creature is often mainly submerged in water. Sounds quite a bit like a hippo.
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This thread needs a bit more controversy, so here's an interesting theory on verse 17. It takes a bit to explain it, and it's not for the squeamish or the easily intimidated.
As is typical of Hebrew poetry, the description of the behemoth consists of couplets: two lines that are thematically linked together. The second line generally re-states or expands in a complementary fashion on what the first line says.
Verse 16 says, "Behold now, his strength in his loins / And his power in the muscles of his belly" (NASB). Both halves deal with strength, and "loins" and "belly" are closely related (loins either refers to the pubic region or hips, such as Genesis 37:34 and Exodus 28:42, or to the centre of a person's strength, such as Proverbs 31:17 and Nahum 2:1).
Verse 18 says "His bones are tubes of bronze; / His limbs are like bars of iron" (NASB). Again, the symmetry between the halves of the verse is obvious. So, we can expect that verse 17 also has this form, and since the verses on each side of it deal with strength, we can expect verse 17 to deal with strength as well.
Here's what verse 17 says in the KJV (I've again added a slash to divide the lines): "He moveth his tail like a cedar: / the sinews of his stones are wrapped together." There's two words that are interesting. First, the Hebrew word translated in the KJV as "stones" only occurs here in the Bible, so its meaning is speculative. In the KJV and other early English translations, it is rendered as "stones", a word which means "testicles" when it refers to a body part (see Leviticus 21:20 and Deuteronomy 23:1 for other examples of this usage). In the Latin Vulgate, this word is translated "testiculorum". In newer translations, it is often translated as "thighs" instead, although there's not much reason for this change, aside from the fact that it's less likely to induce giggles when read aloud from the pulpit. It appears quite likely that this word indeed refers to the creature's testicles.
The second interesting word is "moveth", which in Hebrew is the word chaphets. While this Hebrew word occurs 75 times in the Bible, it is only translated as "moveth"/"sways"/"bends" here. Every other time it is translated as either "delight", "please", "pleasure" or variations on that. In the Greek Septuagint, this word is translated as histemi, a word with a meaning that includes "to cause to make stand", "to make firm" and "to uphold or sustain". In other words, the word chosen by translators thousands of years ago has nearly the opposite meaning of the one chosen by recent translators.
Why don't translators consistently render the word in this instance and instead use a nearly opposite meaning, "moveth"? Perhaps because it doesn't seem to make a lot of sense otherwise -- unless you put all the pieces together. Verses 16-18 all describe the strength of the behemoth. Verse 16 ends with describing the force it has in its loins. In the next verse, the second half talks about tightly wrapped testicles, and we can expect that the first half somehow complements this thought. It refers to a "tail" that is being delighted and pleasured so it is as firm, upheld and sustained as a cedar tree.
What ever could it be referring to?
And, how could this fit into the purpose of God's speech, which was to take Job down a few notches by showing wonders in God's creation that surpass or mystify Job in various ways? Is it at all relevant that male hippos happen to be one of the most well-endowed land animals?
Hmm.
