Creatures known as "Grumpkins" and "Snarks" are considered to be fictitious within the (itself fictional) world of Game of Thrones.
They are supposedly the kind of monsters that little children hear about in nursery stories, the kind spoken of in the same breath as ghosts, goblins, vampires, the bogeyman, etc.
The White Walkers have not been seen by anyone in eight thousand years by the time of the War of the Five Kings, and so they have become the stuff of legend. In the present day, White Walkers are most often spoken of in nursery stories and fictional tales, alongside the likes of ghosts or "grumpkins and snarks".
Thus for characters within the narrative of Game of Thrones, actually encountering the White Walkers is as shocking for them as actually encountering a vampire would be for real-life viewers of the TV series.
In the BooksEdit
Even within the fictional universe of the book series, "Grumpkins and Snarks" are fictional creatures of legend. "Grumpkins and Snarks", almost always mentioned in this pairing, are usually mentioned when people are dismissing the White Walkers as nothing but the stuff of legends and children's tales, like ghosts, mermaids, White Walkers, or Giants.
Curiously, as of the fifth book, A Dance with Dragons, no description or explanation of any kind has been given for what exactly "Grumpkins and Snarks" are supposed to be like. Logically, the characters themselves know what they are supposed to be like, so they don't bother to spell it out in dialogue: it can just be inferred that they're legendary creatures because they're mentioned alongside ghosts and goblins. That is, Tyrion never bothers to explicitly state in dialogue, "Jon Snow, you're off to the Wall to join the Night's Watch to fight Grumpkins and Snarks...which as we all know are a kind of monster with the following attributes..." etc.
Of course, the story reveals that the White Walkers are indeed real, leaving one to wonder just exactly how many of the creatures and races in the legends of Westeros have a kernel of truth behind them.
There is rampant fan speculation that "Grumpkins and Snarks" might really be some kind of minions of the White Walkers, i.e. this might be the proper name given to the giant "ice spiders" which are as big as dogs that the White Walkers command.
|Human races and cultures:||First Men · Andals · Ironborn · Dornishmen · Rhoynar · Hill tribes · Crannogmen · Free Folk· Dothraki · Lhazareen · Valyrian · Ghiscari · Qartheen · Summer Islanders|
|Non-human sapient races:||Children of the Forest · Giants · White Walkers (Wights)|
|Animals:||Direwolves · Dragons · Manticores · Ravens · Shadowcats|
|Legendary and magical creatures:||Grumpkins and Snarks · Harpies · Krakens · Mermaids · Nagga the Sea-Dragon · Shadow · Sphinxes|
|Plants:||Weirwood · Ghost grass|