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. Used in modern English for any huge beast.
Long before Jumbo was dreamed of, a hippo was exhibited by George K. Bailey, who invented the tank on wheels now used so generally in the circuses. The beast was advertised as "the blood sweating Behemoth of Holy Writ," and he made several men rich. [Isaac F. Marcosson, "Sawdust and Gold Dust," in The Bookman, June 1910]