I would give this episode... we'll say 6.5/10. The plot device of one pony being mistaken for another has been used before in other shows, and I'm a little surprised that none of the mane cast thought to ask who Wild Mustang was until the last five minutes.
That being said, I did like how neatly the action of this episode begins and ends with relevant scenes from the previous episode, which means they could be edited together.