Behemoth is probably a rhinoceros or buffalo (or some other plant-eating big mammal in Job’s day). Leviathan is just an ancient near eastern mythical creature. My guess is this is entirely figurative language in the Bible when mentioning Leviathan and it has nothing to do with elephants/sex organs/whatnot. Leviathan is also mentioned outside of Job (in Psalm 74:14; 104:26; Isaiah 27:1). It was a common near eastern tale to say that, in the beginning of creation, there was some sort of primordial chaotic sea, and in this sea, there existed these great divine deities/gods (who are basically sea serpents). Then, as the tale goes, the divine creator(s) comes in and slay the chaotic sea monster, and then make order out of chaos and hence commencing the creation of our world as we know it. The readers of the biblical narratives would have been familiar with these tales circulating around in their days (like the Enuma Elish), and thus would have recognized the motif in the Bible. The primeval history of Genesis (ch. 1-11) clearly take a number of ancient near eastern stories, including Enuma Elish, Epic of Atrahasis, and possibly (not very sure here) the Epic of Gilgamesh, and transforms these stories in order to allegorically set up the narrative of God’s world in a way that the Israelite’s would have been reminiscent of, since they had known these stories, but also in a way that would be knew for them, as the biblical authors transformed these texts in order to allegorically set up the new story being revealed to them with important meanings and motifs that lay the basis for the rest of what is to come.
I recommend watching this Yale lecture. The lecturer takes shots at the Bible here and there but otherwise this is a great lecture on these issues.