Poor Derpy was misunderstood and bullied.
She was born with an out of place eye.
Who cares what you look like? So why should anyone treat Derpy differently?
Just because she got her Cutie Mark for something that she had no idea what it was meant to represent doesn't mean that bullies should bully her.
One day, her past comes back to haunt her and she is forced to leave Ponyville.
How will Dinky cope with her missing mum? And how will Doctor Whooves cope with the loss of his beloved wife?