What Prairie said. You don't have to look farther than she saw an opportunity and took it. You have to take it back. I would warn her (raise the whip and glare and say something in a threatening voice) if she as much as lays an ear back at you, and if she makes a move, get after her but good. Make her feel very sorry she ever had that thought. Then go right back to treating her sweet. But keep your eye on her. Don't let anything slide, ever, and you'll soon have your docile pony again.
Horses (and ponies ) are not machines, tHey don't stay the same, far as work ethics and respect, unless that handling that produced that 'sweet, compliant horse', remains constant
If you don't lead, they will, it is in their very nature as a herd/prey species
Horses, some more than others, constantly ask, 'are you still the leader?'
If that answer is not a clear , yes, they will start to lead.
That does not mean you have to be abusive, control the horse out of fear, but it does mean that you must be a firm , fair and clear leader.
If this is not done, then well trained horses can become 'un trained
Just a personal choice, but I never had my kids learn to ride on ponies. This was due to several things, first, we were raising horses, and not ponies.
I could ride those horses first, make them solid, and even when my kids were riding them, get on and keep them 'tuned'
Not saying there are no good ponies, but the fact remains that they are mainly ridden by kids, thus get away with stuff, and being pretty smart, learn to take advantage