As a fan of the show, I always loved Rainbow Dash and her cutie mark, but I never wanted it to appear on my leg! Now that it has, all sorts of things are getting weird around here. I'm a guy, not a mare! Cartoon ponies aren't real... right?
A portal on Earth is recently discovered leading to another dimension entirely. A strange land inhabited by even stranger creatures. And for the humans, that's... really lame, actually.
A pegasister and her friend get ponified and thrown into Equestria. But they have no cutie marks. (Starts between Season 1 and 2, follows canon with some additions of my own part)