In form it is a plural of בהמה, but in Job 40:15-24 the word is accompanied by verbs and adjectives in the singular (describing a big and powerful beast).
This fact can be explained either be assuming a pluralis excellentiae, a method of expressing greatness by pluralizing a noun, or by assuming a word in the singular.
The second approach is taken by the Midrash (Leviticus Rabbah 13:3), describing the בהמות as a great beast, and probably also by the Peshitta, transliterating the word rather then translating it: ܒܗܡܘܬ.
It has been suggested by modern scholars to be loanword from Egyptian peḥemau, an unattested word that would have ment “ox of the water”, i.e. hippopotamus.
- (Biblical Hebrew) Meaning unknown, perhaps a water ox, that is, the hippopotamus or Nile horse.
- behemoth, a great and mighty beast.