Porpoise

Porpoises appear only briefly in the first movie, How to Train Your Dragon.

Description
Porpoises are mammals that live exclusively in the water and belong to the Family Phocoenidae. They are generally smaller than dolphins and generally inhabit cooler waters than dolphins. Like other cetaceans, porpoises form complex family pods and communicate with clicks and whistles.

The porpoise seen in the How to Train Your Dragon Film is of a dark color with a distinct white underbelly. This may be intended to be a Harbor Porpoise (Phocoena phocoena) or perhaps a Dall's Porpoise (Phocoenoides dalli). The Harbor porpoise is more likely, because these have a distribution range around Scandinavia, while the Dall's Porpoise lives in the Northern Pacific.

Function
Porpoises are only seen to be used as dragon food, specifically for the Red Death.

In actuality, Vikings may have taken advantage of beached cetaceans or herded cetaceans into shallow water to their demise. There is little evidence Vikings took part in full-scale whaling seen in later ages. Blubber, meat, and bone were highly prized.

How to Train Your Dragon
While Hiccup and Astrid are riding Toothless through Helheim's Gate, a Monstrous Nightmare is seen carrying a porpoise. One is seen again being dropped into the volcanic Dragon Island, as many dragons are dropping animals of every sort into the volcano to feed the Red Death.