Hippopotamus

Hippopotamuses, or Hippos for short, are seen once in the How to Train Your Dragon and pointed out in the accompanying film commentary with Chris Sanders and Dean DeBlois.

Description
The hippo that received a cameo in the first film is actually a tribute to the Hippo from the DreamWorks Animation movie, Madagascar.

In reality, Hippos (Hippopotamus amphibius), are the second largest living land mammal after the elephant. They are usually grey in color and have large mouths with canine tusks. Despite their rotund barrel-shaped body, they are actually rather aggressive creatures and can run at 19 mph on land. However, hippos prefer to stay in and around water, and are even considered to be semi-aquatic. The hippo's main diet is plant material, mostly grasses, which they come out of the water at night to graze upon.

Function
Though a cameo appearance from another film, one hippo is seen in the first film as food for the Red Death.

In actuality, historic Vikings would not have come across Hippos, as they are native to mid to southern Africa. They have been hunted and eaten by humans.

How to Train Your Dragon
While Hiccup and Astrid are riding Toothless through Helheim's Gate, a Monstrous Nightmare is seen carrying a hippopotamus to Dragon Island as a food tribute to the Red Death.