It only appears once in the Bible, in Iyov 40:15
הִנֵּה־נָא בְהֵמוֹת אֲשֶׁר־עָשִׂיתִי עִמָּךְ חָצִיר כַּבָּקָר יֹאכֵל׃
Take now behemoth, whom I made as I did you; He eats grass, like the cattle.
This is the opening verse of a section describing this mighty beast (continuing until 40:24). At first glance, it might seem that behemot is the plural of behema בהמה - "animal, beast." And in fact, behemot as the plural of behema appears in 14 other biblical verses.
The problem is that in this case, in Iyov, the word refers to a single animal, very likely the hippopotamus, not a collective of animals. So what's happening here?
There are two theories.
One is that behemot does derive from behema, in what Klein calls "plural extensivus" . This is a phenomenon in many languages, including Hebrew, where to indicate an extension or increase in size or scope, a plural is used when referring to a singular object. We discussed a similar phenomenon here (referring to the names of God), and here explaining why Yom Kippurim is in the plural. According to Fox (here), a good translation would be something like "super-beast." This is also the position of Kaddari, who doubts that the behemot refers to a hippopotamus due to the mention of a large tail (like a cedar) in Iyov 40:17.
The other theory is that despite the obvious similarities between the words, behemot and behema are not cognate. This is mentioned in the Online Etymology Dictionary entry for behemoth:
late 14c., huge biblical beast (Job xl.15), from Latin behemoth, from Hebrew b'hemoth, usually taken as plural of intensity of b'hemah "beast." But the Hebrew word is perhaps a folk etymology of Egyptian pehemau, literally "water-ox," the name for the hippopotamus.
This is also the position of Steinberg in his "Milon HaTanach", BDB, and Tur-Sinai in Ben Yehuda's dictionary, who finds support in the commentaries of Ibn Ezra and Ralbag.
But Klein writes that "the assumed connection of Hebrew behemoth with Egyptian p-ehe-mau, 'ox of the water', was justly rejected by W. Max Muller."
And yet, Slifkin, in Sacred Monsters, writes that "it seems overwhelmingly likely that the account of the behemoth in the book of Job refers to the hippopotamus" (p. 185) and in response to those like Kaddari who have a problem with the mention of the tail, writes that "that it stiffens its tail, which is only likened to a cedar in terms of its stiffness, but not in its overall dimensions. The hippo's tail is less than a foot long, but it is broad and stiff" (p. 187).
So who's right? I know it's cliché, and I sound like the rabbi in that old joke, but I think they're likely both right on some level. Gesenius wrote that "it is probable that the form בְּהֵמוֹת [behemot] really conceals an Egyptian word, signifying the hippopotamus, but so inflected as to appear Phœnicio-Shemitic." In other words, when the speakers of Hebrew first encountered a huge animal called pehamau, and thought it sounded very similar to their existing word behema - they connected the two. This happens all the time when languages meet. (We saw a similar case in our discussion of hodu.)
So while perhaps if we had a time machine we could find a more precise explanation of the development of the word, but until one is invented, I think both explanations are legitimate.