I ride my pony in a 16" Stubben youth dressage. She is a 14hh Welsh/Arab X. I have found that my 16.5-17" saddles are overwhelming looking on her. Too much flap, etc. It does depend on the pony. However, if her back is long enough, could you maybe buy a 17" with a short flap? Maybe a full size dressage saddle could fit, since the flap doesn't come forward?
PS: If you wanted to try the youth saddle route, I have a youth dressage saddle for sale cheap. Not my Stubben, an old English make that was for my larger "pony." :)
Tug on anything at all and you'll find it connected to everything else in the universe.~John Muir