If the pony's only problem is excess hoof length, a farrier can work miracles in a trim or two. But my worry would be if he/she has laminitis in addition to excess hoof length. That is common in ponies and their feet may never be completely normal again.
So I guess it is a case of is it merely excess length or something worse? If you had pictures I'm sure everyone could give you a good idea of what you are dealing with.