Zebra

Zebras are mentioned only once and not seen in the game, School of Dragons.

Description
Zebras are horse and donkey-like mammals belonging to the Genus Equus. They are widely known for their contrasting stripes all over their bodies, usually black on a white base. The stripes serve as camouflage and to disorient predators by making the zebra appear indistinct or appear confusing while in motion.

Function
Zebras have no use in the How to Train Your Dragon Franchise, other than as a memory phrase for the player to recall dragon species in the former Fear Class. "Fear Holds Zebras Still!".

In reality, Zebras are often kept for entertainment purposes, and hunted for food and their hides. Attempts have been made to domesticate them, however they have a "wildness" - unpredictable nature - that hinders efforts.

School of Dragons
Zebras are mentioned in a phrase to remember dragon names of the former Fear Class in the player Quest, "History of Dragons".

Trivia

 * In the How to Train Your Dragon Book Series, there is a species of dragon called the Zebramount, which has stripes like a Zebra.