Whales are marine mammals mentioned in the original How to Train Your Dragon Book Series.
Whales are marine mammals grouped under the term Cetaceans along with dolphins and porpoises. Currently there is not a specific taxonomic group including all species that are generally referred to as whales.
Blue Whales (Balaenoptera musculus) are a particular species mentioned in the Book Series. Blue whales are the largest animal at this current age on earth. The largest recorded specimen was 98 feet long and about 190 (US) tons. Blue whales can occur in oceans worldwide, but after advances in whaling in the 1900's, their populations plummeted to about four isolated groups.
Blue whales do not have teeth, but instead have hard plates called baleen, that act as filters. The whale takes in a large volume of sea water, and strains it through the baleen, leaving behind krill, the major component of its diet.
Another species mentioned is the Killer Whale, also known as the Orca. Despite having "whale" in its name, the killer whale is actually in the Family Delphinidae, or oceanic dolphins. However, the terms "whale", "dolphin", and "porpoise" are more layman's terms than actual taxonomic designations. The killer whale is known for its sleek black body with distinct white markings. It is a toothed whale that hunts fish, other sea mammals and birds, and even other whales and dolphins.
Narwhal are a unique mid-size whale, due to the single long tusk erupting from the snouts of males. These Arctic toothed whales are a mottled grey on their backs and have a pale belly. Narwhal were extensively hunted no only for their blubber, but also their tusk.
Whales are only mentioned in the Books, and serve no function, other than for size comparison and a source for names.
Professor Yobbish's book, Looking After Your Killer Whale suggests they are kept as pets.
In reality, however, Vikings and many other cultures both historic and current, hunt whales for a variety of products they provide. The flesh is edible, the oil used for lamp fuel, and the bones and other parts used for tools and other items.
The blue whale is mentioned in this book as part of the Viking Song that Stoick the Vast sings. One is also seen in the sea as Hiccup steers a Viking ship home. A Narwhal is also seen in the waters at this point, as well.
Blue whales are used for size comparison to Sea Dragons.
|“||Preposterously huge Sea Dragons that were twenty times as big as the blue whale and who killed for the fun of it.||”|
|— How to Train Your Dragon|
|“||Vast ... Gigantic. Stupendously enormous. Five times as big as the Big Blue Whale.||”|
|— Old Wrinkly about Seadragonus Giganticus Maximus|
Stoick the Vast's ship is also called The Blue Whale.
A killer whale is mentioned, which happens upon the sleeping Green Death in the ocean and realizes he is nearing awakening.
|“||But the previous week, a Killer Whale who had chased some seals unexpectedly deep was surprised to notice a slight movement in the upper eyelid of the dragon's right eye. An ancestral memory stirred in the whale's brain and he swam away from there as fast as his fins could carry him.||”|
|— Book 1|
Killer Whales are mentioned again, this time as the subject of one of Professor Yobbish's reference books, Looking After Your Killer Whale.
Whales of no specific species are mentioned when describing the overpowering smell of the Green Death's breath:
|“||It was the stench of DEATH itself - a deep, head-spinning stench of decaying matter; of rotting haddock heads and sweating whale; of long-dead shark and despairing souls.||”|
|— Book 1|
Again, whales are used as a metaphor, this time to describe the behavior of Dogsbreath the Duhbrain, when he is sparring with Hiccup during a 'Swordfighting at Sea' lesson.
|“||Dogsbreath staggered to his feet, as mad as a harpooned whale.||”|
|— Book 2|
Killer whales are mentioned when describing Baggybum the Beerbelly.
|“||...Baggybum the Beerbelly was about the size of a killer whale who hadn't had much exercise recently.||”|
|— Book 2|
|“||The sword was now just an extension of him arm, like the horn of a narwhal is an extension of its nose.||”|
|— Book 2|
|“||At first he though it was a great whale or shark, but eventually he realized it was something far, far worse. It was a tremendous Sea Dragon called a DOOMFANG.||”|
|— Book 4|
Later on in the epilogue of Book 4, an older Hiccup recounts that something large like a whale or shark was following his own ship. It turned out to be the very same Doomfang, but instead followed Hiccup and his ship like a guardian angel. Hiccup also recounts that this enormous dragon played with his ship, which, Hiccup notes, is "common enough behavior in dolphins, and even in humpback whales, who are fascinated by boats". Doomfangs typically ignore humans.