Not all the dragons in the how to Train your dragon franchise are large, many are dog sized or smaller. The smallest are the insect-sized nanodragons but today we are going to look at those species commonly kept as pets and hunting dragons.
The most common in the movies is the Terrible Terror, a breed the size of a large house cat. The most common in the books is the dog sized Common garden and Basic brown breeds. The last is the Horrible Horror, my own hunting dragon according to This Website.
These small dragons may not be ridden but they can carry messages for the PCs (or NPCs for that matter), act as scouts, help hunt down dinner, be familiars, or simply be the team pet.