O, so you espied a big rat and you run straight to this compendium to see if it is some sort of mythical creature? Try leaving the tavern alley and doing some actual adventuring before rushing to the literature. Mayhaps it is just a very large rat!! And what if it IS indeed an unnatural beast, what then? Is this the work of arcane mischief? Who knows. Listen, this is not ballistic science. I'm just telling you what you can likely surmise yourself - the "giant rat" is, in EVEN MORE plain words, a big-arse rat. Stab it until it dies.