My Little Pony: The Mentally Advanced Series is an American abridged animated television series, which was developed in 2011 by Greg of FiMFlamFilosophy. The series of videos recontextualize episodes from the original series, My Little Pony: Friendship is Magic, by re-editing and re-dubbing them.
Episode 11 was meant to be a parody of My Little Pony episode "Fall Weather Friends", but wasn't continued because there was no dialogue to be taken advantage of (dialogue was one of the biggest things Mentally Advanced Series is known for) according to Greg, the creator of the series.
Eventually, after about a year, Greg decided to make Episode 12, which was Suited for Success. The video included a rapid thirty second montage from the lost episode so people acknowledge it's existence. It has been shown twice, during a livestream and once at a con.
In October of 2014, YouTube user chaoko99 had taped the evidence of it's existence from the livestream mentioned earlier, and can be seen in the video above.
Some people claim that "this episode doesn't exist", as the title of Episode 12 suggests, but in fact it does. It was just never completed.