I was thinking about the phrase "I could eat a horse" today for whatever reason, and it occurred to me that its implied meaning and possible origin might be a bit darker than the sense it's normally used in. The thing to ask is, why a horse? Why not some other large animal that people normally eat, that it would be an exaggeration to say you were hungry enough to eat in one sitting, like a cow, pig, goat, etc? Maybe because horse is usually worse meat, which adds to the exaggeration of hunger, but I don't think that's it.
It seems to me that the hunger that the phrase is describing isn't the modern sort where most people can't go 4 hours without getting hungry, or even really a direct description of hunger at all, but rather is referring to the desperation that comes with deciding to kill and eat your farm work animal (and using that link to ordinary hunger as a little joke). Nearly all of us are the decedents of peasants who at some point in our ancestry would have been starving to death and forced to eat draught animals, most of us within the last 200 years. A phrase like "I could eat a horse" (if wasn't already an omnipresent cliche) just wouldn't be interesting enough to gain currency if it wasn't making a clever little connection to some other concept than the feeling of hunger. It's a bit interesting to think about how these things get lost over time, even if it makes no difference now.