I know not the answer you are looking for - but buy a cheaper nice pony and train it for 6 months, showcase the pony and sell for more money. This will demonstrate your commitment as well as help you put more money down on the $$ horse.
So them his qualities what are point out the all the bad things and compare them to one or two good qualities he has. Then talk about what you like in him and then ask your parents what they like about him.... Hope this helps and hope you get that pony. Good Luck to you.