Combined with the stop and rear…this goes beyond a quirky head toss, sounds like a training problem. You put the rider best able to deal with it on the Pony, not your friend. You should see if the guy will work with you on the Pony or, maybe, find it another owner-horses are expensive and if you don’t get along with one but others do? Something to consider.
Been around a few German Riding Ponies brought over as show Hunter Ponies, positively spectacular in the ring but…under the right rider, not an average child, even if they were pretty good. Hence they aren’t that popular a choice over here.
I mean this in the kindest way…they are very German in attitude. Stubborn too. You click or you don’t and if they know you aren’t sure or are scared, you are in trouble. Obviously the guy is in tune with this Pony and has his respect while too many other riders didn’t command that respect from the Pony and created some obedience issues. Maybe that head tossing stunt too if Pony knows it bothers some riders.