The Behemoth is a monster that no one dared to challenge back then as it was a colossal monster that could destroy a kingdom if it rampaged. This is true because of the sheer size that this monster becomes as the largest one recorded was the size of a two-hundred-meter mountain. Just a stomp from the largest Behemoth could squash a village which is why people immediately run away when they encounter one.
Behemoths are feared monsters but there was one that was slain before and recorded in history. A powerful dragon slayer once killed a fifty meter tall Behemoth and presented its heart to the king of a kingdom that is severely ill. The king of that kingdom thanked the dragon slayer and had a feast with the Behemoth's heart as a dish.
Edited. Apologies for the late edit as my head hurts today.