Rocket-Rages are dragons mentioned three times in How to Seize a Dragon's Jewel, though none of the three passages gives an outright description or information about this species.
There is no verbal or pictorial description of Rocket-Rages. However:
|“||He was also attacked on the way by three Brainless Leg-Removers and two smallish Rocket-Rages, which he had to fight off all on his own.||”|
|— Snotlout having been left to his own devices on the Amber Slaveland sands|
This verifies that Rocket-Rages are indeed dangerous, but also small, since one human can fight them off. At least, they would probably be smaller than a man.
|“||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|
Again, Rocket-Rages must be on the small side in order to infiltrate a heavily guarded castle. It also means they have the capability of being stealthy. The fact that Hiccup thinks these dragons could single him out from the others and carry him off (rather than eating him on the spot) also lends to the assumption that Rocket-Rages were intelligent enough to do so, and were aligned with the Dragon Rebellion.
Rocket-Rages are mentioned but not described in Book 10.
|“||There could be Brainless Leg-Removers down there ... There could be Rocket-Rages ... There could be something even worse than these ...||”|
|— Hiccup's thoughts while out on the sands of the Amber Slavelands|
This passage tells us that Rocket-Rages live beneath the sands of the Amber Slavelands, and can be very scary and dangerous for humans.