Hero's Spirit
In Twilight Princess, the Hero's Spirit, also called the Hero's Shade, is a skeleton-like undead figurehead that teaches Link each of the seven hidden skills throughout the game.
The Golden Wolf
The Golden Wolf, also called the White Wolf, is the animal incarnation of the Hero's Spirit. The wolf is summoned by finding a howling stone, with the exception of the first time, in which he confronts Link directly. Once a howling stone is found, the song must be howled in tune with the Spirit. He will then tell you to "take sword in hand, then find [him]," implying that Link should return to human form before searching for the Wolf. The Golden Wolf will also appear as a symbol on Link's map, which is useful in case the player forgets where the location of the Hero's Spirit is. When he confronts the Wolf, Link will be transported into an ancient ice arena. The wolf will finally transform into his true form, the Hero's Spirit, who will then teach you one of the seven hidden skills.
The Skills:
Note: At the end of his teachings, he says he accepted the role as the Hero long ago. This implies that he may be the Link from Ocarina of Time. This theory is also supported by the hero's Spirit saying this to Link: "Go and do not falter, my child!" However, this is mearly theory and has not been proven as fact.