As we know, Doflamingo is actually a world noble. And I thought of it, Brownbeard mentioned that a world noble gave the Punk Hazard dragon a name. Also, Momonosuke saw a hallucination of Doflamingo in his eastern dragon form. Since the dragon and the fruit was artifically made by Vegapunk, perhaps Doflamingo found a way to force him to work for him? That dragon was made to guard Punk Hazard, so perhaps Doflamingo made Vegapunk create the dragon so no one will find Caesar Clown. If you have any ideas, comment them. This definitely makes perfect sense.