Some people use the "green" tripe of beef, lamb or elk in stead of veggies/fruit. And although I haven't done a lot of reading on the Prey model, that would make sense, as the tripe is the stomach lining which apparently wild carnivores eat first. My girls love tripe...I haven't yet met a dog who doesn't...but the smell is very very off putting for the humans
My raw food store sells the green tripe, raw for dogs/cats...but I've also served Tripet canned tripe, it's very good for picky eaters in general.