The Hogfly is a minor character that first appeared in the book, How to Betray a Dragon's Hero.
When Hiccup saw Snotlout on the chunk of ice floating on the river, there is a Hogfly beside Snotlout. Hiccup saved Snotlout and made friends with the Hogfly. Hiccup later used him to find the King's Lost Things, which includes Toothless.
He is a very small dragon with big nostrils used for great sense of smell. He can also puff up in order to minimize injuries.
Hogfly is dumb, so he does not understand what is happening, since he often gets lost in what is happening. This is normal for his species.