The Terrorfang and Terrorwing are both mentioned once in How to Ride a Dragon's Storm, a few chapters apart. It is possible that they were meant to refer to one particular species of dragon, but between one chapter and the next, the name got changed slightly. Or perhaps they were indeed meant to be two separate dragon species. Neither name derivation has any information of description beyond those two passages. It can be assumed that the dragon(s) are Sea Dragons, are big, and are dangerous.