Sandsharks are a dragon in the How to Train Your Dragon Book Series, first described in Book 12. They are aligned to the Dragon Rebellion, but are considered to be one of the less fearsome rebel dragons.
Because of its aquatic like appearance and ability to 'swim' through the beach sands, the Sandshark might be classified as a type of Sea Dragon.
Sandsharks are depicted as smallish dragons about the size of a large dog with black or dark coloration and four legs with webbed feet. They are called Sandsharks because of the sail on the tops of their heads that greatly resemble a shark's dorsal fin. Their tails also end in a fork, which is different from most dragons, which typically have a spade.
Most notably, Sandsharks are able to 'swim' through sand like water, with only their head sails showing. They also shoot sedating darts from their mouths.
These pack hunters shoot poisoned darts at their targets, by lifting their heads from the sand just long enough to fire, then duck back under the surface. The darts are poisonous, but usually only bring about unconsciousness, rather than death. The larger the prey, the more darts are required to bring it down. The subdued prey then put up no resistance as the Sandsharks rapidly devour it.
Sandsharks appear in the last book of the series. A pack of them attack Hiccup and the Hogfly after he washed up ashore on an island near the Island of Tomorrow, with no memory of who he is or what his tasks were.