There was a sudden and sharp pain in dash's back and she began to groan in agony.
"What the hay is happening to me?!" She screeched as her bones crunched and snapped, changing. "AHHHG!!!" Rainbow howled as strange, dragon-like wings extended from her body and blood trickled down from her wings. Dash fell to the ground, crying and panting. "Why....??" She whispered to herself.
You can say that's an isolated incident. I didn't really think of any bigger story to that, but the thing that makes her change is a curse or some spell maybe, a nasty stuff that she got into by an accident.