It's not a very elegant prize, but I suspect somebody will want to study it. If you'll excuse me...

[ Kojirou squats down and experimentally hefts the rat a few times before successfully getting half of it over his shoulder. He can just about drag the body around like this. He knows a scholar who would probably be very interested in what would cause a rat to grow bigger than a man, and also be practically immortal. ]

I'm not the most sensitive to such things, but it is very thick, isn't it? That might explain the rats.