It is very durable (most hits won't even penetrate its hide to actually harm it) and has lots of HPs. Best to summon AFTER you get endgame sword. And you are lucky. If you summoned the deadly Ylside woman instead, you would have died just by approaching her. You can beat her only by trapping her on the opposite side of the goblin city gate, throw one or two fireballs on her face from afar and give her 10-30 hits with weapon. Just in case you get that unlucky summon instead of the rat. She drops secret items, though, which are very powerful.