Can someone help me identify this episode?

Hi all,

I can't quite believe I'm doing this, but I'm trying to track down the name of a particular episode of this show - in fact the only episode I have any memory of, but one that was a part of my first childhood memories of television.

Okay, first of all, there was this evil cat-like villainess, and her sidekick/boyfriend (?) this lizard-creature. I recall a song he sings telling how she used to be good, but is now bad.

At the same time, one of the ponies is feeling depressed because she's only good at jumping which keeps getting her into trouble. However, by the end of the episode she leaps over some sort of pit to save a foal that's about to fall in.

Lastly, there is some sort of costume party going on, and one of the foals is sad because she doesn't know what to go as: the human girl dresses her as a princess and as the finishing touch she gives her this magic necklace that she wears. The evil cat and the lizard disguise themselves as a pony dressed as a nurse and trick her out of it somehow.

And that's all I remember, but I really want to track down this episode, or at least know the name of the episode and what season it was in.

Thanks!