I have been going to Goodwill stores for years, but I have never once found a single pony-related item at any of the place I have been to. But that's my luck. My brother worked at Goodwill for a while, and they let him buy stuff all the time. When he picked me up from school occasionally, he would have some DVDs or other things he got for half off because he worked there (the rule was that you had to put it on the shelf first, but at the end of your work shift you could grab something!).
I think it really depends on the store, too.
EDIT: That's interesting, Sarahlacewing... I guess the store my brother worked at was corrupt. Muahahaha!!