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.
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.