The Carrier Dragon is mentioned only a couple times in the How to Train Your Dragon Book series.
There is no description given of this dragon, but it is small, as it can fly into a home. The Carrier Dragon's usage and name brings to mind carrier pigeons. Most likely this dragon is on the small side, used by Vikings to deliver letters.
Though not explicitly mentioned, the Carrier Dragon must be relatively easy to train. It is used as a messenger of letters and mail between the tribes, particularly the Bog-Burglar Tribe and the Hairy Hooligan Tribe.
A Carrier Dragon is mentioned once, carrying a letter from Big-Boobied Bertha to Stoick.
- "In the middle of the meal a Carrier Dragon flew in the window, dropped a letter addressed to Stoick on the table and flew out again."
- ―Book 3
Later, Stoick sends a Carrier Dragon back to Bertha.
- "So Stoick had sent a Carrier Dragon with a (very polite) message to Big-Boobied Bertha suggesting that they talk things through."
- ―Book 3