TL;DR: 6yo green mare throws the middle hoof and bolts back to the barn almost without warning while being led. Many ideas tried/considered. How fix?

So...to set the stage.

We've got a handful of homebreds on the farm here, one of whom is from the daughter of one of my parent's former polo ponies. She's a TB/RID IDSH cross and somehow missed the "horse" part, which is totally fine by me since I like my horses small (she's 15.1, maybe .2). What she did get is the cobbish barrel and chest; she's basically built like a mini tank. I absolutely adore her, because a) I loved (and love) her mama and b) she clearly skipped a generation and got her granddam's personality (and also maybe inherited the polo gene)--sassy but sweet and so insanely clever.

...Too clever for her own good, sometimes.

She's had this one problem most of her life. Not exactly sure what or how it started, but as young as like...a 2yo? Anyway--she will, no warning (often some warning but I kid you not sometimes it's like a lightning strike), shoulder check you and absolutely haul ass back to the barn and her friends. (We're on roughly 15 acres and the ring is probably 200 yards from the barn, it's all a pretty compact cozy set-up.) She did that once to me when she still had the habit of throwing a double-barrel at whoever she knocked over and I'm pretty sure her hoof whizzed right past my head. Thankfully, she's knocked off that part of the habit.

We've tried so many things. In regular work--she makes the attempt anyway, to varied degrees of success (usually stopped with my dad; I can stop her, but last week she got away from me about 4 times). Rope halters (✔️). Nose chain (😬). Lunging/work near the barn and her friends so that doesn't become the getaway destination (the work/rest idea; granted, haven't been doing that for long since we just got back to nice weather again). Warwick Schiller's program (CAT-H, +R, energy work, ish). Flexion (working on her response to pressure that will and does translate to rein aids). Doesn't seem to be medical-related. She even has attempted it with a bridle (she's in a leather bit to start, I think she's had a metal one in before but not since she's been backed); it does help her not to get completely away, buuuut...y'know...bit pressure...

She does sort of have similar moments on the lunge line sometimes when she gets on too big a circle? Not so much the sudden jerk and bolt but just drifting out until she tries to say "adios". Seems to be particularly prevalent when there's not as much tension on the line either (not pulling tension, really, just holding her to the circle).

Besides the dreaded nose chain, keeping her attention while on the lead seems to be the most effective way of just heading it off before it starts, but...that just invites constantly choking up on the lead and doing the racehorse leading thing, which I've personally been working to get away from the last couple years. She leads perfectly fine 99% of the time between her paddock and the barn (probably all of 100 feet). If her buddy is in the stall next to her, she's usually pretty calm. We've done a lot of work to get her a little less feral-acting, and she's come a long way, but she still throws a fit when her friend goes away or she has to go away from her friend.

I've been just working on changing her focus if I can be in the stall with her while the friend goes to get worked (she can literally see her friend from the barn....... smh they're so silly sometimes), and we're 99% sure most of the issue is something to do with separation anxiety or just a general "F you I don't wanna do what you're asking me" (my mom's opinion; I think it's all the same basic source problem and that starts with the anxiety), but we don't have a lot of options for addressing that piece. Which is also kinda funny because the one or two times we've taken her friend to go trailer to various trainings, she gets tossed in with the other youngsters, and it's as though her friend didn't exist.

We're planning to see if maybe our local trainer has some insight to offer, but it's to the point where she's too unsafe to take off property (she loads into the trailer brilliantly, because she's so clever, but otherwise hasn't been off the property her whole life). I feel that if she could just learn that there's all kinds of fun things we can do together if she doesn't pull away, she'd have a ball going out and about. I don't want to assume, of course, anthropomorphizing or anything, but...

Is there anything more we can try?

I'd carry a lunge whip with you so when she does this, if you can get her in time, when she shoulder checks her I'd get her backing up immideately

However if it happens too fast I'd just try to set your weight when she runs off and then spank her butt with the lunge whip and then start lunging her pretty hard make her MOVE

If she's such a big fan of running make her do it until she's not a fan lol

also get her reversing directions and really listening to you because she's not being respectful of your space and boundaries

Horses, especially young ones get anxious when it isn't clear that you're leading them and calling the shots

shes anxious without a buddy because she feels alone and unsafe because she doesn't see you as the leader

Also I would really recommend getting a trainer to help you with this it's way better to have someone there with you to teach and correct on the spot and as it happens!