More then a hundred years ago - after the battle which created the City of the Swamp, a crocodile, already large by any account, devoured a nearly-destroyed 'thing', one of the weapons of either the city, or it's enemy.  This was not the end of it, and instead of simply becoming food for the great beast, it joined and fused with it.

It weakness had shattered its mind, preventing it from taking over the croc. Instead, the being exists as a passenger within the Croc, seeing and experiencing everything it does.  It does lend a degree of supernatural power to the Croc, which is how it has survived in the ruins of the city for so long.  It is one the reasons The Pack is no longer as powerful as it once was, attacking from below the water and dragging pack members under.