We have a 12.1hh 21 year old loan pony which came to us about 3 months ago. To start with, she was pretty perfect, but in the last few weeks she has ditched my 8 year old DD 3 times - every time she was cantering around and the pony dropped a shoulder and did a sudden sharp spin to the side.
The vet and chiropractor have seen her and said they can find nothing wrong. We changed her saddle when she first came but it was fitted by a saddler and no problems for the first couple of months - just in case, we changed back to her old saddle last weekend but she did it again! So I don't think it is that (although we have a new saddler coming this week to check).
I now think she is just being naughty - her owners said she did something similar at her loan home about 10 years ago with a fairly weak rider but has been good as gold since (as verified by her most recent loan homes).
The only other things that have changed recently are that she's now shod on all fours (although has been shod in previous homes) and we switched from night-time to day-time turnout about a month ago.
My plan is to get my daughter (who is fairly competent for her age) to ride her a bit more bossily, with a shorter rein, more defensive seat, etc., and take things right down a level (no cantering for a while), and I am giving her a quick 20 minute schooling session myself a couple of times a week. I'm planning to have my hat with me next time DD rides and if she does it again then get on and give her a good telling off and make her canter around and around until she is tired, so she hopefully learns bad consequence happen when she does it (I haven't told her off yet as I was worried it might be pain).
Does anyone have any other ideas?! Or encouraging stories of how they fixed their pony's bad behaviour? I really don't want to give this pony up as when she isn't doing it she really is a fantastic little pony but my daughter is fairly nervous and although she's got back on every time she's fallen off so far, I can see a point coming where she will just lose all her confidence.