The Flightmare is a Mystery Class dragon released on August 6th, 2014. It can be purchased for 750 gems (600 for members).
As for V2.3.0, in the intro sequence, the Flightmare is one of the four Secondary Starter Dragons that can appear caged, most likely to unlock the Stable Missions without needing to buy a new dragon (the player still has to choose their actual starter dragon).
"The Flightmare is a medium-sized dragon that has a very ghost-like appearance. Its body is covered in tiny spots that almost resembles the night sky.
"From what has been seen, the Flightmare is a very aggressive and territorial dragon, being very protective of it's territory and food sources. Seeing how Flightmares are build for speed and defense, they usually avoid fights with bigger and stronger dragons. Flightmares have been shown to be quite determined, as the Flightmare in Fright of Passage kept chasing his opponents, and was willing to search food over long distances. They seem to be quite reclusive and avoid contact with humans, unless, they get in the way of their algae, or if they think that they do."
For more information on the Flightmare, click here.