Oh boy, I created a monster, I knew I shouldn't have mentioned it...
It seems to be one of those harmless urban legends that becomes "fact" because of how harmless it is. It's a good story, so why not let it be true?
The story came from this: The carousel has several different "levels" of horses -- "A" horses are the really big and grandly decorated ones, that you see on the outer ring. As you move to the inner rings, the horses become smaller and less decorated, and are called "B" horses, "C" horses, etc. referring to the level of detail and size. (Apparently the "A" horses are VERY expensive, hence why they can't make them all A's). So one day there was a girl visiting MK with the 'Make a Wish' foundation, who was in really poor health. She wanted to ride the carousel, but they had to help her in the process. Due to her condition, they needed to place her on one of the inner rings, on one of the smaller horses -- but the poor girl wanted to ride one of the "A" horses on the outside. So a Disney cast member made up the story of Cinderella's horse on the spot to make the girl feel more special for riding that particular horse. The horse was the smaller inner horse with the golden ribbon, and the cast member made up the story that this was Cinderella's horse, and she was special for being able to ride her horse.
After this, the story spread and became "fact" -- and even as this lady admitted, it's a harmless story that's able to be used to explain to smaller kids when riding this horse, so the story stuck. I mean, it's all made up at some point anyway
I guess it doesn't matter where the story came from, right?