Deer are mentioned in several books of the How to Train Your Dragon Book Series.
Deer are 'even-toed ungulates', meaning they have and even number of toes (two) on each foot that are hooved. Deer occur in North America, South America, Europe and Asia, however, there is only a small area in Africa where they occur and none in Australia or Antarctica. Deer species found in Scandinavia include: Moose - also known as Eurasian Elk (Alces alces), Roe deer (Capreolus capreolus), Red deer (Cervus elaphus), Fallow deer (Dama dama), and Reindeer (Rangifer tarandus).
The primary use of deer in real life and in the Book Series is as a food source for Vikings and Dragons. Other parts of the animal can be used as well, such as the hide for making leather.
Reindeer have been domesticated by the native Sami people of Scandinavia, who rely on herding reindeer for survival. The reindeer are used for transportation, food, milk, and crafting various useful items.
|“||Hiccup had once seen a deer that had wandered too close to Wild Dragon Cliff torn to pieces in a matter of minutes ...||”|
|— How to Train Your Dragon|
Later on in Book 1, deer are mentioned again as prey animals:
|“||If you had been a rabbit or a deer you would not have noticed them until you felt the talons on your back and the hot fire on your neck.||”|
|— About Dragons hiding in the bracken|
|“||They had grabbed each other by the shoulders and were carrying out a staring contest, the horns on their helmets locked together like a couple of rutting stags||”|
|— Book 2|
A "stag" is an adult male deer.
A stag is said to have become incapacitated by Big-Boobied Bertha's bosom.
|“||Fists like sledgehammers, ears like cauliflowers, she had once stunned a stag with one blow of her mighty bosoms, and many a smaller animal had suffocated in their stern depths.||”|
|— Book 3|
It is possible some of these stags appear later in the book as part of a feast in the Hysterics' Great Hall.
|“||The long central table was loaded high with fish, flesh, and fowl cooked in every possible manner, whole stags, entire pigs, and brimming cups of beer and wine.||”|
|— Book 4|
Also in the Hysterics' Great Hall is a "map of the Barbaric World drawn on deerskin".
Reindeer feature prominently in the beginning of Book 5. The Viking students have Hearding-Reindeer-on-Drgonback lessons during the summer on Huge Hill. Toothless causes the herd to scatter by chasing them rather than herding them, Eventually the boys and dragons gather the reindeer and drive them toward Hooligan Village. However, both the boys and the reindeer are surrounded by fire caused by Exterminator dragons, who slowly kill the reindeer.
Reindeers are mentioned in the description of the Vampire Dragon.
|“||These nocturnal bloodsucking creatures will attack any large mammal--reindeer, sheep and even humans.||”|
|— A Hero's Guide to Deadly Dragons|