Something of monstrous size, power, or appearance.
(Middle English, from Late Latin, from Hebrew bĕhēmōth)
Bible authors borrow from popular contemporary imagery for stylistic impact.
They want to make a specific point but to do it with evocative overtones, not because they’re showmen or shrill but because they have a crucial central theme. They need to ensure message recall. And to do that, the message needs to resonate with the reader (or listener) — so that it will be imbibed and, hopefully, taken heed of.
That, after all, is the role of the prophet: to get God’s message out.
And so then …
But even in Job (see below) there is the suggestion at least of the spatial approximation to a river:
in Arabian mythology, Bahamut or Bahamot is a vast fish that supports the earth
Let’s now look at a few different sources — two popular secular sources, and the Book of Job from the Bible:
The Merriam-Webster says:
The original behemoth was biblical; it designated a mysterious river-dwelling beast in the Book of Job. Based on that description, scholars have concluded that the biblical behemoth was probably inspired by a hippopotamus, but details about the creature’s exact nature were vague. The word first passed from the Hebrew into Late Latin, where, according to English poet and monk John Lydgate, writing in 1430, it “playne expresse[d] a beast rude full of cursednesse.” In English, behemoth was eventually applied more generally to anything large and powerful.
In Job 40:15-24, we read:
15 Behold now behemoth, which I made with thee; he eateth grass as an ox. 16 Lo now, his strength is in his loins, and his force is in the navel of his belly. 17 He moveth his tail like a cedar: the sinews of his stones are wrapped together. 18 His bones are as strong pieces of brass; his bones are like bars of iron. 19 He is the chief of the ways of God: he that made him can make his sword to approach unto him. 20 Surely the mountains bring him forth food, where all the beasts of the field play. 21 He lieth under the shady trees, in the covert of the reed, and fens. 22 The shady trees cover him with their shadow; the willows of the brook compass him about. 23 Behold, he drinketh up a river, and hasteth not: he trusteth that he can draw up Jordan into his mouth. 24 He taketh it with his eyes: his nose pierceth through snares.
And finally, over at Wikipedia, this:
In Jewish apocrypha and pseudepigrapha such as the 2nd century BCE Book of Enoch, Behemoth is the primal unconquerable monster of the land, as Leviathan is the primal monster of the waters of the sea and Ziz the primordial monster of the sky. According to this text Leviathan lives in “the Abyss“, while Behemoth the land-monster lives in an invisible desert east of the Garden of Eden (1 Enoch 60:7–8). A Jewish rabbinic legend describes a great battle which will take place between them at the end of time: “…they will interlock with one another and engage in combat, with his horns the Behemoth will gore with strength, the fish [Leviathan] will leap to meet him with his fins, with power. Their Creator will approach them with his mighty sword [and slay them both].” Then, “from the beautiful skin of the Leviathan, God will construct canopies to shelter the righteous, who will eat the meat of the Behemoth and the Leviathan amid great joy and merriment.“
A hungry hippo?
With that said, let me leave you with a redacted piece about the humble hippo (Wikipedia):
Despite a mostly herbivorous existence, the common hippopotamus (ancient Greek for “river horse”) is — after the elephant and rhinoceros — the third-largest type of land mammal and the heaviest extant artiodactyl and a highly aggressive and unpredictable animal — ranked among the most dangerous in Africa. Furthermore, despite their physical resemblance to the pig and other terrestrial even-toed ungulates, the closest living relatives of the Hippopotamidae are cetaceans (whales, porpoises, etc.).
Common hippos are recognisable by their barrel-shaped torsos, wide-opening mouths revealing large canine tusks, nearly hairless bodies, columnar-like legs and large size; adults average 1,500 kg (3,300 lb) and 1,300 kg (2,900 lb) for males and females respectively. Despite its stocky shape and short legs, it is capable of running 30 km/h (19 mph) over short distances. (Nevertheless, they are still threatened by habitat loss and poaching for their meat and ivory canine teeth.)
The common hippopotamus is semiaquatic, inhabiting rivers, lakes and mangrove swamps, where territorial bulls preside over a stretch of river and groups of five to thirty females and young. During the day, they remain cool by staying in the water or mud; reproduction and childbirth both occur in water. They emerge at dusk to graze on grasses. While hippopotamuses rest near each other in the water, grazing is a solitary activity and hippos are not territorial on land.