"Derpy Hooves" is a character that, before that episode, was exclusively a background character that didn't speak and wasn't named. She had crossed eyes (some say it was originally an animation error, some say it was an intentional easter egg) and appeared overall fairly clumsy and awkward, in a sweet and humorous way.
The community created their own character out of this, including a peculiar love for muffins, a job as a "mailmare" and a daughter. She's really popular, almost rivaling the main characters.
Supposedly, the creators has acknowledged that Derpy was popular in the community and has included her in several episodes just for the hell of it. This is more pronounced in season 2.
In "The last roundup" she is at last referred to by name as "Derpy" (the name she was supposed to have was supposedly "Ditzy Doo", and she is referenced as this in "Winter Wrap Up" but not visible at the time) and given a voice. Reception was kind of mixed but mostly positive.
A few days ago the episode mysteriously disappeared from iTunes and a lot of Derpy merchandise was suddenly unavailable. There was a lot of concern that that some people found the character offensive and Hasbro had decided to chicken out and remove Derpy from the show. Come yesterday(I think, not very long ago though) the episode reappeared with this edited scene.
Original version of the scene:
http://www.youtube.com/watch?v=dCUT-KHWeec
Changes that I noticed: Name removed (that part of the dialogue sounds really awkward now), Eyes underped, voice changed to more feminine (I can kind of understand this as Tabitha herself has said she mistakenly thought the character was male. I didn't really like the original voice that much either, but it is decisively worse now IMO).
