Extreme polyphagia is a very rare condition, one where the person suffering is unable to control their appetite, and in some cases will literally eat anything. One of the most famous historical cases, and certainly one of the most disturbing, is that of French soldier and sideshow performer Tarrare, who lived from approximately 1772 to 1798. 
He was turned out of his house at a very young age because of his ravenous appetite and found work as a traveling performer, known for eating massive quantities of rocks, corks, and live animals in front of a crowd. At the age of 16, he moved to Paris to be a street performer and made his living eating strange and disgusting objects. Shortly after joining the French Revolutionary Army at the outbreak of the Revolutionary Wars, Tarrare was hospitalized, where he was experimented upon by Dr. Courville and the head of surgery, Baron Percy. They fed him live animals and massive meals, monitoring his digestive process and how he reacted to various foods – a live cat, live eels, snakes, puppies, and more were offered to him by the doctors, and Tarrare gobbled them up without satiating his hunger. Below is an account of Percy’s observations from Anomalies and Curiosities of Medicine by George M. Gould and Walter Lytle Pyle (1996):
Percy saw the famous Tarrare, who died at Versailles, at about twenty-six years of age. At seventeen he weighed 100 pounds. He ate a quarter of beef in twenty-four hours. He was fond of the most revolting things. He particularly relished the flesh of serpents and would quickly devour the largest. In the presence of Lorenze he seized a live cat with his teeth, eventrated it, sucked its blood, and ate it, leaving the bare skeleton only. In about thirty minutes he rejected the hairs in the manner of birds of prey and carnivorous animals. He also ate dogs in the same manner. On one occasion it was said that he swallowed a living eel without chewing it; but he had first bitten off its head. He ate almost instantly a dinner that had been prepared for 15 vigorous workmen and drank the accompanying water and took their aggregate allowance of salt at the same time. After this meal his abdomen was so swollen that it resembled a balloon. He was seen by Courville, a surgeon-major in a military hospital, where he had swallowed a wooden box wrapped in plain white paper. This he passed the next day with the paper intact. The General-in-chief had seen him devour thirty pounds of raw liver and lungs. Nothing seemed to diminish his appetite. He waited around butcher-shops to eat what was discarded for the dogs. He drank the bleedings of the hospital and ate the dead from the dead-houses. He was suspected of eating a child of fourteen months, but no proof could be produced of this. He was of middle height and was always heated and sweating. He died of a purulent diarrhea, all his intestines and peritoneum being in a suppurating condition.
The most disturbing detail of Tarrare’s short life is that hospital staff believed that he had eaten a 14-month-old toddler who disappeared during his stay there. He was chased out of the hospital, although he was never formally charged with the murder. In the last few months of his 26 years on this planet, he suffered from advanced tuberculosis. Years of abuse left his body in an unbelievably putrescent state, and when the doctors opened him up after his death, they found his organs swimming in pus, and his stomach riddled with ulcers. Tarrare has gone down in history as one of the most terrifying cases of polyphagia in human existence.
Sources:
Anomalies and Curiosities of Medicine, George M. Gould and Walter Lytle Pyle (1996)
“Tarrare,” wikipedia
