§ Of Beren and Lúthien The Silmarillion

But Huan the hound was true of heart, and the love of Lúthien had fallen upon him in the first hour of their meeting; and he grieved at her captivity. Therefore he came often to her chamber; and at night he lay before her door, for he felt that evil had come to Nargothrond. Lúthien spoke often to Huan in her loneliness, telling of Beren, who was the friend of all birds and beasts that did not serve Morgoth; ad Huan understood all that was said. For he comprehended the speech of all things with voice; but it was permitted to him thrice only ere his death to speak with words.