Most probably, behemoth is plural for behama, meaning an animal in Hebrew, suggesting that the creature is as big as several animals.
It is impossible for anyone to kill a behemoth except for the person who created it, in this case Jehovah. Jewish tradition holds that at the banquet at the end of the world, the behemoth will be served up along with the leviathan and ziz. The Hebrew behemoth is equated with the Persian Hadhayosh, as the leviathan is with the Kar and the ziz with the Simurgh.
Metaphorically, the name has come to be used for any extremely large beast.