Rúmil (Music of the Ainur):

Rúmil, one of the residents of the Cottage of Lost Play, admits to having spent time in the captivity, or as he called it, thralldom, of Melko, which he used to learn the languages of the monsters. Apparently, he had an older appearance, with grey hair.
Along with the languages of the monsters, Rúmil said that he had learned the languages of the Valar and the Eldar, and the languages of the birds and beasts. However, he said that Mens speech changed too often for him to learn it well. In adition to his skills with language, Rúmil was a noted storyteller.

His captivity was recorded to have happened at some point before the fall of Gondolin, and to have lasted for a long, though unknown period of time.

The Book of Lost Tales One (HoME.I)
The Book of Lost Tales Two (HoME.II)
The Lays of Beleriand (HoME.III)
The Shaping of Middle-Earth (HoME.IV)
The Lost Road and Other Writings (HoME.V)
The Return of the Shadow (HoME.VI)
The Treason of Isengard (HoME.VII)
The War of the Ring (HoME.VIII)
Sauron Defeated (HoME.IX)
Morgoth's Ring (HoME.X)
The War of the Jewels (HoME.XI)
The Peoples of Middle-Earth (HoME.XII)