Eels are long snake-like fish from the Order Anguilliformes, comprised of about 800 species worldwide. Most are carnivores of some type. Eels are typically a food source for humans, but must be cooked prior to ingestion as their blood is toxic.
Conger eels are a group of Eels in the Genus Conger. Historical Vikings may have encountered the European conger (Conger conger), which occurs along the Eastern Atlantic coast line from Norway down to the Mediterranean Sea. European congers can grow over 6 feet long (females) and are carnivores, eating other fish and crustaceans. They are grey in color with a pale grey-white belly.
Electric eels are also mentioned in Hiccup: The Viking Who Was Seasick, or rather they are implied. Though electric eels are very long snake-like fish, they actually belong to the Order Gymnotiformes, not Anguilliformes. Therefore they are not actually eels, but are mentioned here to avoid confusion.
In the Book series, eels are mentioned as a descriptive term or seen in the background.
In reality, eels of many species are eaten around the world. Due to over-consumption, as well as damage to the environment, the European Eel is critically endangered. The electric eel is so-called because it can produce bio-electricity from specialized organs.
Hiccup sees "eels that lit up like lightbulbs" while he steers a Viking ship home after a big storm at sea, in Hiccup: The Viking Who Was Seasick.
|“||Gorgeous silver cups with seahorses galloping delicately around them and golden necklaces as plump as oysters and swords as sharp as a conger's tooth with octopus tentacles winding themselves around the hilt.||”|
|— Book 2|
|“||You can't keep a Bog-Burglar under lock and key. No prisons can hold us - we're as wriggly as eels.||”|
|— Camicazi in Book 3|