The same thing happened to one of my quail a week or two ago. I had a ground pen of 5 or 6 quail and something must have gotten to it overnight because the next morning I found one the exact same way, except the feathers were still there. Giant hole from the backside to below the ribs, and the insides were completely cleaned out. Everything else was untouched and so were the other quail. I’m thinking it was either a rat, possum, or weasel since those are really the only predators that could have made it into my pen and would eat a quail like that. It wouldn’t have been another quail, it’s not very common for them to eat each other, especially not like that. Also they would have blood around their beaks or breasts when they’ve been attacking/eating each other.