Nanodragons are a small breed of dragon.
In The Books
In How to Speak Dragonese, the Fat Consul ate them as a snack. By switching one of the nanodragons for an Electricsquirm and electrocuting the Fat Consul, Hiccup rescues one who calls himself Ziggerastica and claims to be a god. Later on, Hiccup calls on Ziggerastica to rescue him and his friends from the Romans. By floating in the air with the help of several nanodragons, Hiccup convinced the spectators that he was the god Thor and got them to leave.