Jell-o. When your done, you can eat it.
Okay, seriously, I was thinking about this last night myself. Plain gelatin, mixed thick should work fine. I think ballistics gelatin is great for seeing how a bullet performs, but I think it is too dense to represent how it performs on an animal.
Although, my mind did wander to the idea of going to a local pig farm and buying 1 or 2 whole hogs, dead of course, but not gutted. And again, when your done, you can eat it.