I've been doing a bit of reading in Job, and there was one thing I found somewhat humorous, yet also interesting, to share. However, the topic delves into "mature" subject matter, so please take your own sensitivity into consideration before continuing onward. (Trust me; one wouldn't think there is anything particularly "mature" regarding the behemoth... but we'll get there.)
I place this here under Bible Chat because, after all, this is a discussion on the Bible, and a matter of interpretation. This is really an issue of non-importance, but it's an interesting issue nonetheless, which is why I place it here for consideration.
Regarding the behemoth, there are two common conclusions regarding its identity. First, it is often assumed to be a hippopotamus. The second, however, is that it is a dinosaur. This seems to be a belief I run into more and more in the Christian circles I traverse, but I'm not entirely sure what the more common belief is in Christendom as a whole. But, if Answers In Genesis is an accurate summary of the majority, it's one I would like to challenge in the following exegesis.
The behemoth is found in Job 40.15-24, where God seeks to challenge Job. Many of the descriptors found in this passage are readily comparable to the hippopotamus, but a few specific statements cause others to believe that the text is instead referring to a brachiosaurus. One of the most commonly cited reasons is found in Job 40.17, which is commonly translated as something similar to the following: "He makes his tail stiff like a cedar; the sinews of his thighs are knit together." [ESV] This verse, it is claimed, shows that the behemoth's tail is as big as a cedar tree, and hence could not be a hippo's tail. Likewise there is verse 24, which says that the behemoth cannot be captured; hippos have been, dinosaurs have not been.
But even these simple reasons I find room to challenge. So here are the following reasons as to why I believe the behemoth may be a hippopotamus.
First. Several of his habits and bodily features are immediately comparable to the hippo. He eats grass similar to an ox.  He has incredible strength in his body.  He has very strong limbs.  His skin is quite tough. 
Second. His dwelling place is near rivers, and marshes. [21, 23] He is able to stand firm against the river's current.  Again, this readily matches the hippo.
Third. Verse 16 describes how the behemoth's power is in his "navel". The Hebrew word used here is shariyr, which is given as "cord" [Mickelson]. It is derived from the word shor, which specifically means "a string, i.e. the umbilical cord" [Mickelson] or "umbilical cord, navel, navel-string" [BDB]. I do not believe that the Bible is a scientific treatise, but it would be odd to find a statement that a dinosaur (being a reptile, which is born from an egg and has no umbilical cord, nor navel) should be described as having a "navel".
Fourth. In verse 24, God asks Job, "Can one take him by his eyes, or pierce his nose with a snare?" This, as stated above, is a common objection to the behemoth being a hippopotamus, and that it is instead actually a dinosaur. But it would be an unnecessarily over-literalization of the question to insist that God is claiming that no one has ever or will ever overcome the behemoth. For one, it would be contradictory to James 3.7, where the Apostle states that "every kind of beast and bird, of reptile and sea creature, can be tamed and has been tamed by mankind".
So which is it? Has the behemoth been overcome by man or not? This leads to the conclusion that God's questions towards Job are strictly rhetorical, not word-for-word factual. For instance, in Job 39.1, God asks Job, "Do you know when the mountain goats give birth? Do you observe the calving of the does?" Obviously men had already and have since observed mountain goats give birth, and have kept track of their pregnancies. The point is that, what man (being finite) struggles to do, God (being infinite) does effortlessly.
Hence, God is not claiming that man had never and could never overcome the behemoth, but merely that what man struggled to do (overcome the behemoth), God could do with ease if he so chose to do. Hence, the behemoth could yet be the hippopotamus.
Fifth. The behemoth is not described as being terribly large in size. It is big, but not monstrous. Verses 21-23 present him as relatively squat. He is able to be shaded by lotus plants, and he is able to hide between marsh reeds. Also, the "Jordan rushes against his mouth". Even if he was a dinosaur, the behemoth certainly would not be a brachiosaurus; his face is regularly level with the river-water or marsh-water, which is far more fitting of the hippo.
Sixth. The common English rendition of verse 17 is quite misleading, to say the least. I will provide here the ESV translation:
He makes his tail stiff like a cedar; the sinews of his thighs are knit together. [ESV]There are several issues to pique here. The first is one that should be most obvious. The text is not describing the size of the behemoth's tail as being "like a cedar". It is describing the action of "stiffening" the tail "like a cedar". If I describe a friend's acne as being as pimpley as as basketball, it would be a huge error for someone to interpret me as claiming my friend's face as a whole is as big as a basketball as well. Nothing in the text requires that the behemoth's tail be as big as a cedar tree, only that it is able to stiffen it like a cedar tree.
Seventh. However (and here comes the "mature" part), another issue seems to be that "tail" is not the best interpretation of the Hebrew text. Before we directly address that, let's first turn to the second line of the text: "the sinews of his thighs are knit together". What the ESV gives here as "thighs", is the Hebrew word pachad. It means "testicle", "as a cause of shame" [Mickelson].
Now, there is a common aspect of Hebrew poetry we recognize as parallelism. Meaning, two consecutive lines in a poem or poetic statement contain the same message. That is, Line 1 gives such-and-such statement, and Line 2 repeats it in another way. We find this parallelism evident in the preceding and following verses of Job 40.17. Verse 16 contains two parallel lines that describe the behemoth's bodily strength, and verse 18 contains two parallel lines that describe the behemoth's limbs.
Verse 17 also contains parallelism. The second line of this verse refers to the behemoth's testicles. (The KJV renders it as "stones"...) As such, we should expect that the first line contain a parallel element, which the Hebrew calls the "tail". Biologically, what element of the (male) body is parallel to the testicles? It appears, then, that the Hebrew is calling this male appendage a "tail" only in euphemism. (Deuteronomy 23.1 calls this male appendage a "pipe" in euphemism.)
Eighth. Lastly, there is a particular verb at issue in verse 17, the Hebrew chaphets. In every other instance in the Bible, this verb translates to "delight" or "pleasure" or something related. In this one case, for no reason at all, most English translations give it as "to move" or "to bend".
To recap the previous points: The second line of verse 17's parallelism directly refers to the behemoth's testicles. The first line refers to a "tail"-like appendage that, as we see here in Point 8, is a "delight" to the behemoth; a "tail"-like appendage that becomes stiff like a cedar tree.
In no way to be crude, but I think it's obvious that what is being described to us, this "tail"-like appendage, which is parallel to the testicles, which the behemoth takes "pleasure" in, is not really a tail.
Conclusion. Everything in the text readily indicates that a hippopotamus is being described. Nothing here can really be stated to resemble a "dinosaur", but all of it can easily be corresponded to a hippopotamus.