I could eat a horse

From the Inky Fool:

What with the news that almost every snack in Europe is actually my little pony, and the jokes about spaghetti bologneighs, I keep being asked about the origin of the phrase I could eat a horse. Specifically, does it mean:

1) I am so hungry that I could eat something as large as a horse, an elephant or a blue whale.

Or

2) I am so hungry that I would be prepared eat something unusual, like horse, squirrel or cockroach.

So I set off to trace the phrase back. It turned out to be popular all the way through the nineteenth century. But once you get far enough, the phrase changes to I could eat a horse behind the saddle. 

 So, it's not the largeness but the lack of appeal that is unusual in the phrase.

Also: "thirsty enough to drink barley-water."  Or, to use a modern equivalent, Cherry Pepsi.