Frogs are mentioned in the How to Train Your Dragon Book series, but not actually seen.
Frogs (Order Anura) are amphibians who do not have a tail in their adult form, though larval stages - known as tadpoles - typically do have tails for swimming. Despite being bound to water or moist habitats, frogs occur worldwide with the exception of the Arctic, Antarctica, and true deserts. Though the greatest species diversity occurs along the equator in rain forests, frogs that live in cold and dry climates have many adaptions for survival. Many species are able to enter a suspended state of torpor or hibernation. During winter in many areas, frogs may enter hibernation in a burrow, in mud at the bottom of a pond, or under leaf litter. Other frogs can even be frozen, keeping vital organs from freezing by an excess accumulation of glucose. Some frog species in dry climates undergo aestivation (hibernation in hot weather) after burrowing into the earth and adapting physiologically.
In the Book series, frogs are mainly used as a derogatory descriptor for dragons. However, at least one human is referenced eating frogs.
In reality, the primary use of frogs is as a food source for humans. They are also used in medical research and medicine.
|“||Something that makes a Monstrous Nightmare look like a flying frog!||”|
|— Fishlegs hoping Hiccup grabbed a "cool" dragon from the Dragon Nursery|
|“||What is it, Hiccup? A brown bunny rabbit with wings? A flower fairy? A fluffy flying frog?||”|
|— Snotlout describing Toothless|
Gobber refers to Hiccup and Fishlegs as "tadpole poos" when he sees their boat sink and return without completing a 'Board-an-Enemy-Ship' lesson.
|“||AS IT IS, YOU ARE THE MOST USELESS, MISERABLE, PATHETIC EXCUSES FOR TADPOLE POOS I HAVE EVER MET IN MY LIFE!||”|
|— Book 3|
|“||Oh look, it talkssss ... The little Human tadpole is speaking the noble Dragon tongue ...||”|
|— One Eye in Dragonese|
Then Snotlout mentions frogs when making fun of Toothless because he is wearing a winter coat.
|“||'What is your ridiculous frogspawn of a reptile wearing?||”|
Later, the intense green color of Doomfang eyes is described in depth, relating them to the color of green frogs.
|“||It was as if all the green in the world, of peas, of grass, of spinach, of leaves, of beans and frogs, had been concentrated in that one eye and given it the intensity of pure green acid.||”|
|— Book 4|
Frogs are referenced again when Camicazi scales the wall surrounding Hysterical Village, "appearing to cling to it with invisible suction like a frog."