I dont know that anyone could write an add for you. Adds are supposed to explain all the great things about the horse being advertized. That goes from training, showing, whether it can clip, load. Is it good with the farrier? etc etc. Only you would be able to do that, or someone who knows the pony specifically.
Also keep in mind that when posting for the asking price, the horse market has been in the dumps for months so starting off a little lower than what you want, might be your best option to sell successfully.
I suggest going online and reading what others have put up and put an add together from there. Once you have something together, then you can always come here for someone to edit, but no one here can make an add for you kiddo.