Howling Stones are rocks from Twilight Princess where Link can howl as a wolf. There are seven total. All are used to sing duets with the Hero's Spirit and learn the location of the Hidden Skills, except for the one in the Sacred Grove, where Link howls Zelda's Lullaby. All of the songs from Howling Stones are from previous games in the series, aside from the very last one.
When Link howls at one of these stones, he is transported to a strange location, where the Hero's Spirit's golden wolf form is. He must howl the song a second time and then perform a duet. Once he finishes and returns to Hyrule, a symbol of the golden wolf will appear on his map. If he goes to this symbol, he can learn a Hidden Skill.