|“||He was so scared he could hardly think. How could he be attacked from inside the castle? What could this be? His mind jostled with Rocket-Rages, with Piranha Hermit dragons, with something worse than all of these . . .||”|
|— Hiccup after being abducted within Prison Darkheart|
No description or information is given about this species. Though unusual, drawing conclusions from its name are rather vague. Does it look like a piranha? Or does it act like piranha? Both? But instead of shoals, are they loners, hermits? Or is hermit referring to a hermit crab? In which case does this dragon have a shell? Does it move from home to home as a hermit crab does with shells? We may never know, and the author isn't telling.
Piranha Hermit Dragons are mentioned briefly as part of the strange anti-human dragons that can be found in the Amber Slavelands.