depending on what it is, it can count under either a parody or semi-original content (such as FCs and stuff). If it's fan art, then that's a different story. Mostly, though, it's used with stuff that earns money.
In short:
If it makes profit: Not Ok.
If it's a joke off of the show or a song of the show/a picture or such of the likes: Ok.
If it's made by you and is used in the fandom but not actually in the series: Also Ok.
I don't know why this was vetoed and not the thousands of pictures of ponies being sold online... You'd probably have to go into the legal system and such if you wanted that info.