The Collaborative International Dictionary of English v.0.48:

Behemoth \Be"he*moth\, n. [Heb. behem[=o]th, fr. Egyptian
   P-ehe-maut hippopotamus.]
   1. An animal, probably the hippopotamus, described in --Job
      xl. 15-24.
      [1913 Webster]

   2. something of large size or great power.
      [PJC] Behen

WordNet (r) 3.0 (2006):

behemoth
    n 1: someone or something that is abnormally large and powerful
         [syn: giant, goliath, behemoth, monster,
         colossus]
    2: a person of exceptional importance and reputation [syn:
       colossus, behemoth, giant, heavyweight, titan]

Easton's 1897 Bible Dictionary:

Behemoth
   (Job 40:15-24). Some have supposed this to be an Egyptian word
   meaning a "water-ox." The Revised Version has here in the margin
   "hippopotamus," which is probably the correct rendering of the
   word. The word occurs frequently in Scripture, but, except here,
   always as a common name, and translated "beast" or "cattle."


Hitchcock's Bible Names Dictionary (late 1800's):

Behemoth, beasts