Gaelm has told you that you can exchange your Bree-pony for another.
'Greetings, I am Gaelm, son of Éogar.
Have you chosen a pony for yourself instead of a horse? A Bree-pony would buckle under the load of someone of your stature! You must exchange it for a Bree-horse.
I would ask that you be careful though in your chose this time. My father already looks down on these Bree-steeds...I would not wish him to think those that ride them have addled wits.'
Gaelm is at the Hengstacer Farm, north of the Bree Festival Grounds.
If you wish to trade in your pony for a horse, you should speak again with Gaelm.