I could eat a horse

Of course, nobody ever actually ate a horse; not all at once, anyway, which is what is suggested by the phrase.

Actually, an estimated 3 million horses have been butchered in the USA in the past 10 years for human consumption abroad. Apparently, it is considered a delicacy in countries such as France and Belgium. So when some people say, "I could eat a horse", man, they mean it.