Paragraphs, man, paragraphs.
That said, tail is a euphemistic word used in place of penis. Key contextual points being how the tail stiffens, how Behemoth's strength is in his loins, and the mention of his stones. It's pretty obvious that tail refers to penis in this case. Behomoth itself is likely a bull.
Leviathan is obviously based on exaggerated accounts of crocodiles. I shouldn't have to explain why.