The Dragon That Lived Under Manhattan: 11/17/16
The Dragon That Lived Under Manhattan by E.W. Hildick is about a knight who comes to Manhattan to rescue a dragon he believes is trapped under the streets. He's helped by a boy who has made a wish to meet a real knight and through just the right set of circumstances has been able to have his wish granted.
I found the book while looking for titles to add to my "Under Manhattan" list of fiction (books, TV, films) that take place beneath the surface of New York (discounting typical under ground usage, like riding the subways). There is a beast (Beauty and the Beast, 1990s TV), ghosts (Ghostbusters 2, turtles (Teenage Mutant Ninja Turtles), zombies (Blue Moon), man-sized insects (Mimic), but I'd never heard of a dragon before. Chronologically, though, The Dragon that Lived Under Manhattan comes first, being published in 1970.
Interestingly the dragon's territory is very similar to that of the insects in Mimic. The old pneumatic line is too tempting a locale to leave alone. It's just sitting down there in a hazy splendor awaiting rediscovery either in real life or in the collective imagination.