Eagles are birds of Prey mentioned very nominally in the How to Train Your Dragon Book series.
Eagle is a loose term to describe about 60 species of birds of prey within the Family Accipitridae, and are otherwise taxonomically unrelated to one another. Accipitridae also includes birds of prey referred to as hawks, buzzards, and vultures. Eagles generally - but not exclusively - refer to very large, solid birds of prey with blocky heads and sharp, curved beaks characteristic of all birds of prey. Eagles actively hunt, but also eat carrion. Specific diets vary, ranging from fish and Waterfowl, to snakes, deer, and even monkeys.
Interestingly, the image of an eagle in How to Speak Dragonese appears to be a Bald Eagle, which is native to North America, not Europe.
In the Book series, eagles are mentioned as metaphors to describe something else. They are the symbol for the Roman Empire, also in the Book series.
In real life, however, eagles have a variety of cultural and spiritual symbolism for many cultures around the world. In Norse mythology, eagles were associated with blood, death, and battle. Eagles have also been used for falconry, and even used by humans to hunt grey wolves.
The term eagle is used to describe Toothless as he sits on an upturned helmet floating in the sea.
|“||... crouched in an upturned Outcast helmet like a malevolent eagle sitting on a nest, his eyes glowing like candles.||”|
|— Book 2|
The Eagle is mentioned and appears multiple times in Book 3 as the symbol of the Roman Empire.
|“||In time to another trumpet fanfare two slaves brought in the Ceremonial Shield of the Legion, gorgeously decorated with a golden eagle carrying a fish.||”|
|— Book 3|
In the Epilogue of How to Speak Dragonese, Hiccup relates that he still has some items from his time at Fort Sinister, even though he is elderly at that point.
|“||Even the quill with which I am writing now is made out of a Roman golden eagle's feather that I found in my cell at Fort Sinister.||”|
|— Hiccup in Book 3|