Heinrich is a frequent name for an Elbe (sprite), house spirit or poltergeist in ancient German mythology. The diminutive form Heinz is also commonly associated with these spirits, which are almost exclusively male. The house ghost of folktales is often a talkative, inquisitive fellow, who is friendly, well-meaning but irritating. Such a spirit is often encountered as a cold and clammy presence but usually not seen. He is described as having thin hands like a frog, with cold, moist skin. Like the little Frog King, these ghosts often demand to sleep in the same bed as household inhabitants, insist on a place at the table and want to eat the same food. They assist in doing household chores but are best known for offering unwanted and sometimes comical advice. Once entrenched, it is extremely difficult to get rid of these spirits.
As more and more people in Europe were Christianized, the old deities that lived in water wells or inhabited trees may have gradually come to be considered rustic, unsophisticated or even powerless. The new faith needed to be appealing to potential converts; it offered a vision of justice, forgiveness, redemption and eternal life. The princess in the story rejects the frog king, whose realm is that of water sprite or house spirit. In a fit of irritation, she attempts to smash the little frog and destroy all that he represents. This absolute rejection of the old faith, magically transforms it and yet preserves its most sacred elements.
Iron Heinrich is a more mysterious and complex character. When taken out of his pagan milieu, he is very puzzling indeed. Germanic tribes believed that every person possessed a good and bad angel, (not unlike the later concept of the lower or higher angels of our nature). These spirits brought about good or created evil for their masters. In The Frog King, it is the evil spirit or hex which transforms the prince into a frog. Iron Heinrich, it would seem, is the higher angelic being, interested in preserving and saving the prince. These benevolent angelic beings were apparently thought to be connected to each person with bands or chains that could be severed only by death. By the same token, the malevolent angel could only be subdued by being chained to a pole. At the conclusion of the fairy tale the bands that connect Iron Heinrich to the prince are heard breaking. In the end, the redemptive power of love has prevailed and brought about a startling transformation.
The heart bound by chains is a powerful image. I am not aware of any sources indicating that this symbol was common in ancient mythology. However, it is reminiscent of the Christian symbol of the sacred heart of Jesus, which conveys the idea of death and redemption through the power of love.