This dragon was in fact adopted and raised by “Pupa”, Chrysalis’ son (Check out on Vector-Brony), and used as an experiment to become a changeling-like creature. It is also used as a great help on finding large live prey to feed the changelings with Pupa as it’s help. 
Eventually, it is made into a fierce killer and major threat to Midnight Starlight and the rest of the other equines in the Equestrian landscape. But that was only the beginning of it in Midnight’s childhood when it was killed too soon after falling into a large lava pit until it was brought back from the dead by a powerful and evil spirit named “Lord Grogar” in the future chapter stories. 
Mostly, it is only made as a bounty hunter to kill Midnight and the family tribe who serves the realm of light preventing them from trying to achieve and bring light and hope in all of Equestria. 
It’s second secret weapon, besides powerful biting as it’s main course, is to occasionally control any carnivorous animals (Lions, Tigers, Bears, [Oh My :3] etc) as manipulated armies which makes this hybrid and the changelings unstoppable (Unless there is actually a weak spot for Midnight to expose the creature).
 
MLP: FiM (c) Lauren Faust and Hasbro 
Changeling Dragon Hybrid (c) Me