Puff the Magic Dragon

Donna Noble compared the need for the creature in the lassimater to keep the Child of Music alive for so long to Puff the Magic Dragon, when the Tenth Doctor said it couldn't bear to live thousands of years without its best friend. Though the Doctor lied he thought of the same thing, he was really thinking of himself. (PROSE: Most Beautiful Music)

Behind the scenes
Though not elaborated on, "Puff, the Magic Dragon" is a song from the 1960s, about Puff, an ageless dragon, and his best friend, a little boy who grows up and leaves Puff on his own. "A dragon lives forever, but not so little boys."