There are levels of exhaustion that, once reached, can absolutely cause the exhausted to be completely removed from all realms of reason. In such a state, the consumption of an entire horse is certainly possible, however unlikely it may seem. Should you encounter someone in such a state of exhaustion, please do your best to steer them away from horses and towards more mundane sources of sustenance, such as used batteries or old car tires. Doing so will go a long way in preventing the untimely death of our fine equine friends. Thank you.
The full quote is in so tired I could eat a horse behind the saddle.
Essentially you're so hungry you'd eat a horses ass. One would assume horses ass would be rather tough and stringy plus there's the whole that's where poop comes from thing.