"Unicorns have magic, Pegasi have the sky, and Earth ponies have the ground beneath their hooves. Though they are all different, they share a common trait amongst themselves. All of them have the ability to earn a cutie mark. So, where do I belong?"

Soapbox -- a donkey born into a pony family -- tries to understand where he fits in since he can't use magic, fly, have the strength of an earth pony, or earn a cutie mark.