The screen-shot seen below depicts that
this handicapped Pegasus (Stellar Eclipse) is
actually a real original character voiced by
none other than Sylvain Portelance.
- Courtesy from the MLP Wiki page
With the help of Hasbro and the Make A Wish Foundation (not to be confused with the song of the same name from the "Pinkie Pride" episode which was also called 'Make a Wish'), this was a dream come true for Portelance.
For more info on this, click here.
For more info on the voice actor, click here.
Info about Spinal Muscular Atrophy here.
More info on the Make A Wish Foundation here.