Why on earth would "little" not be acceptable as well as "small". RIDICULOUS!! Also, it's very discouraging.
Have you checked for a different error? I think small and little are always both accepted for petit.
A pony is not a small horse. It is a separate species. A miniature horse is a small horse and not a pony.
In the listening exercise, is there any difference between this being "un poney et un petit cheval" vs "un poney est.."?
"et" and "est" do not make the same sound. I encourage you to train your ears in differentiating both.
Also there should not be a liaison after "et".