Iron John
"Iron John" is a German fairy tale found in the collections of the Brothers Grimm, tale number 136, about a wild man and a prince. (The original German title is Eisenhans, a compound of  Eisen  "iron" and Hans, like English John a common short form of the personal name Johannes) It represents Aarne-Thompson type 502, "The wild man as a helper".
Most people see the story as a parable about a boy maturing into adulthood.  The story also became the basis for the book Iron John: A Book About Men which spawned the Men's Movement in the early 90's.

# Synopsis
A king sends a huntsmen into a forest nearby, and the huntsman never returns. The king sends more, each meeting with the same fate, until the king sends all his remaining huntsmen out as a group, but again, none return. The king proclaims the woods as dangerous and off-limits to all.
Some years later, a wandering explorer and his dog hears of these dangerous woods and asks permission to hunt in the forest, claiming that he might be able to discover the fate of the other hunters.  As they come to a lake in the middle of the forest, the dog is almost dragged under by a huge arm. The hunter returns to the forest the next day with a group of men to empty the lake. They find a naked man with skin like iron and long shaggy hair all over his body. They capture him, and where he is locked in a cage in the courtyard as a curiosity. No one is allowed to set the wild man free, on penalty of death.
Years later the young prince is playing with a ball in the courtyard. He accidentally rolls it into the cage where the wild man picks it up and will only return it if he is set free. He states further that the only key to the cage is hidden beneath the queens pillow.
Though the prince hesitates at first, eventually he builds up the courage to sneak into his mothers room and steal the key. He releases the wild man, who reveals his name to be Iron John (or Iron Hans, depending on the translation). The prince fears he will be killed for setting Iron John free, so Iron John agrees to take the prince with him into the forest.
As it turns out, Iron John is a powerful being and has many treasures he guards. He sets the prince to watch over his well, but warns him not to let anything touch it or fall in because it will turn instantly to gold. The prince obeys at first, but begins to play in the well, finally turning all his hair into gold. Disappointed in the boys failure, Iron John sends him away to experience poverty and struggle, but also tells the prince that if he ever needs anything, simply to call the name of Iron John three times.
The prince travels to a distant land and offers his services to its king. Since he is ashamed of his golden hair, he refuses to remove his cap before the king and is sent to assist the gardener.
When war comes to the kingdom, the prince sees his chance to make a name for himself. He calls upon Iron John who gives him a horse, armor, and a legion of iron warriors to fight alongside him. The prince successfully defends his new homeland, but returns all that he borrowed to Iron John before returning to his former position.
In celebration, the king announces a banquet and offers his daughters hand in marriage to any one of the knights who can catch a golden apple that will be thrown into their midst. The king hopes that the mysterious knight who saved the kingdom will show himself for such a prize.
Again the prince asks Iron John for help, and again Iron John disguises the prince as the mysterious knight. Though the prince catches the golden apple and escapes, and does so again on two more occasions, he is eventually found out.  The prince is returned to his former station, marries the princess, and is happily reunited with his parents. Iron John too, comes to the wedding, but now without the hair or iron skin that made him frightening. He reveals he was under enchantment until he found someone worthy and pure of heart to set him free.

# Variants
This tale is known throughout Europe, in such variants as The Hairy Man.  A more wide-spread variant, found in Europe, Asia, and Africa, opens with the prince for some reason being the servant of an evil being, where he gains the same gifts, and the tale proceeds as in this variant; one such tale is The Magician's Horse.
The oldest variant to be preserved is the Italian Guerrino and the Savage Man.  Another such variant is Georgic and Merlin.