Thelma the Unicorn by Alan Blabey

​Thelma is an ordinary pony who wishes with all her heart to be a unicorn. ... Then in a rare pink and glitter-filled moment of fate, Thelma's wish comes true. She rises to instant international stardom, but at an unexpected cost. After a while, Thelma realizes that she was happier as her ordinary, sparkle-free self.

The Return of Thelma the Unicorn