Made by an in-game editor.
To access the editor you have to go to DeviantArt, click download, make the screen bigger in height than width and press home+end
Here is a more detailed tutorial from someone else:
http://goo.gl/ZTJTG

These zombie ponies can't catch me even if i quadruple them.
I added more zombies in more locations to increase the difficulty.

My Little Pony belongs to Hasbro

Friendship Is Magic was made by Lauren Faust 
and Studio B

Story Of the Blanks was made by Donitz

http://browse.deviantart.com/?qh=&section=&global=1&q=story+of+the+blanks#/d3kq34i

Added on 30.01.2013
Personal review:
Should have made it shorter, showing only the major edited parts. The video should have started at 1:42.
I should have also played around with the zombies, juggling around, showing off, like at 00:35 i could have waited a few more seconds before exiting.
I should have not recorded this at 1:00AM,
Should have been far less frantic with the dialogue or simply not show it at all.
Could have shown Twilight's form as a blank, it looks like an old asian-master with the beard.
Should have definitely shown off the editor and how to work with it.
Most importantly I should have learned some programming, saving this version and putting it up on the internet for playing. Or saving the code from the editor and giving it to someone who knows how to do it.
And optionally making a Benny Hill version.