first off... I wouldnt keep the pony... it go bye-byes...
WARNING: NOT EVERYONE WILL AGREE WITH THESE TECHNIQUES!
I have a rearer... I bought him as a rearer at the age of 3 and I had SEVERAL trainers attempt to help fix this issue, and now, at the age of 24, the boy will still rear!
Here's what I've tried (I warn you - some things seem very mean but rearing is one of the most dangerous things a horse can do and that is one vice that cannot be allowed.)
Crack an egg over his head when he's in the air
Break a water balloon over his head in the air
Hit him with a wooden bat in the air (Like a part of a broom handle)
The thought behind hitting him in the air is that he thinks he is hitting his head and is supposed to not want to rear up like that because he thinks there is a ceiling. You have to do it on the way up, BEFORE he starts back down, otherwise it doesnt work.
Use a tie down, it will keep him from getting the height. That's not a fix though. They learn when they are wearing a tie down and when they are not, smart ones will even learn to rear while holding there head close to their chests... (the tie down I ended up using was a thin plastic coated wire and a leather strap that tied to the girth - it was harsh but he got to where he would not rear if he had that on).
The things that actually worked BEST for me were:
When he was in mid air, I would grab the rain and yank his head to my knee. It would knock him off balance and he would be forced to put his feet down. BUT - He got smart, he learned that as soon as I leaned forward to get that rain, he could whip his head the other way and I would go flying out of the saddle with the reign still in my hand!
The thing that really worked the best (other than keeping him moving forward - they can't rear if they are moving forward, although he would rear up and leap forward which was never fun).
But the thing that worked best was to take my knuckle and jam it just above the withers, it obviously causes discomfort because he wouldnt get but maybe 18inches off the ground when I would do that. So that's the method I usually us when he rears... Or I just ridet he rears. But the thing is, they are very, VERY dangerous and I think rearers are kind of like alcoholics. They are never cured. They are just "recovering".... I have great rides w/ no rears and I have monster rides...
SO my final suggestion: Get rid of the pony...
"Be a best friend, tell the truth, and overuse I love you
Go to work, do your best, don't outsmart your common sense
Never let your prayin knees get lazy
And love like crazy"