Eat a horse? I believe this sentiment to be outdated. I don't often see horses, so it doesn't work as a strong metaphor for me. Plus, it would actually be a weird amount of food. Unlike, "I could eat a mountain of potatoes", which is so far removed from reality, that it works better as an exaggeration. My suggestion is to change the former to "I'm so hungry I could EAT HORSE." With that expression, you'd get immediate feedback of "Wow, that's disgusting. You must be pretty darn hungry."