The pony depicted in this picture is Pinkamena Diane Pie, the depressed version of the show's most vibrant and energetic character: Pinkie Pie
she has been long considered violently insane after the epsiode "Party of one", where pinkamena essestially creates pseudo-frendships with inanimate objects and becomes violent with another character: Rainbow Dash.
pinkamena has become arguably the fandom's most recognizable and universal character show probably followed only by the "Dash-tards" that have a strange love for Rainbow dash
[youtube]http://www.youtube.com/watch?v=JHy3ZhY9VKs[/youtube]
which leads us to the final reason this is a creepy wallpaper: Cupcakes
Running off of the coflict that arose in "Party of one" Between Rainbowq Dash and Pinkie pie, many fans decided that this couldn't just end so abruptly, so some sick fuck out there wrote cupcakes. depicted in this video:
[youtube]http://www.youtube.com/watch?v=4oaYkYcYadU[/youtube]
...the creepiness never ends does it?