Might As Well Dance by Grundy, polutropos, daughterofshadows, Anérea

In that hour Huan forsook the service of Celegorm, and sprang upon him, so that his horse swerved aside, and would not approach Beren because of the terror of the great hound.  

Huan sensed the pain of his former master, his friend, when he threw himself between him and Beren. He knew that without Huan at his side Turco would fall to even greater evil, that his anger would consume him. But Huan felt in his spirit, as if Arōmēz himself moved it from across the Sea, that in this quest his doom was written. So his love turned to Lúthien, as Beren’s had; for she drew love unto herself and gave it back as naturally as a tree takes up sunlight and bears fruit. So his loyalty was given to them.

It is told that Huan pursued the sons of Fëanor, and they fled in fear; and returning he brought to Lúthien a herb out of the forest. With that leaf he staunched Beren's wound, and by her arts and by her love she healed him; and thus at last they returned to Doriath. 

It took Beren some time to get used to Huan. He remembered the dogs they had in Dorthonion and none had been quite as large as the hound that once belonged to the son of Fëanor.

Lúthien, of course, had no such concerns. Huan was hardly the strangest animal she had met growing up, for Melian caused and attracted strangeness in turn, and so the hound quickly became another of her animal friends.

Beren befriended Huan during the quiet moments of their quest. There were head scritches and little games, brief moments of happiness set against their dark journey and slowly, they grew closer. And their bond may not have been as quick to grow as that between Lúthien and Huan, but it was no lesser for it.

Beren seeing their approach was dismayed; and he wondered, for he had heard the voice of Tinúviel, and he thought it now a phantom for his ensnaring. But they halted and cast aside their disguise, and Lúthien ran towards him. 

It was a shock when he saw his beloved walk in the guise of Thuringwethil. He had to look hard several times to convince himself it was not truly the vampire come again. Both moved with the same grace that marked them as not quite human - or at least, as daughters of something other than Man. The dark hair, the starlit eyes, the shadows swirling around her… Only when he heard the reassuring song was he certain this was his Lúthien, ready to brave Angband for their love and her father’s folly. Huan did not speak, but he did laugh.

Then for the second time Huan spoke with words;

The great hound’s company had been particularly appreciated on the cold nights as they lay in the crook of tree roots in a wood, or sheltered in a cave in the hills, or out on the open plains, curled up together in the cosiness of his shaggy coat. And although it seemed at times to Beren that Huan would give him the odd knowing look, he had appeared an otherwise ordinary hound. So when Beren first heard Huan speak he was dumbfounded. And dire as their circumstances were then, Beren couldn’t help wondering in the back of his mind (and with a tinge of a blush) just how much of his lovesome whispers to Luthien he had heard! As Huan turned to leave, Beren could have sworn he saw the great hound wink.