"It had been a few months since Twilight gained the focus and confidence for a new large spell. Finally, however, she was able and willing. An alicorn spell. A spell that could give any pony whatever traits it needed to own both wings and horns. It was not easy, nor short; and it needed severe concentration. The slightest distraction, even so miniscule, could ruin everything. She began. Slowly, determined, focused. She closed her eyes and imagined the steps in her mind. Suddenly, something was wrong. Her mind went blank. A step was missing! A bright flash of light shone from her horn and a loud clapping of what could only be compared to thunder sounded. When it was done, everything seemed normal except the stray papers spread across the floor. No wings were protruding from her back. She could only assume that nothing had happened, except that failure of her spell. Little did she know of the changes appearing at Sweet Apple Acres.
Well I wanted to portray an unhappy winged Applejack and and a happy wingless Rainbow Dash, an irony of what would really happen to the characters.
I'm sure Applejack would be happy to have wings and RD would be unhappy to lose them, but I thought, "maybe AJ wouldn't be able to fly well and life would be simpler for RD."