I was watching a TV show the other day and they featured a new type of insulated portable game back. Essentially, you freeze bottles of water (as in soda bottles, etc) and it fits into this flexable insulated bag that folds up relatively small (the size of a suitcase when filled with frozen water bottles). When you kill your deer/hog you gut it out, stuff the cavity with some of the frozen bottles, put the animal into the bag, toss in the other bottles, and zip it up. The expanded cooler bag looks to be big enough to hold a medium sized hog.
Looks like it would work best for someone hunting off of a 4-wheeler where he couldn't hold a standard cooler easily.