Just my two cents on all this:
So…the mare obviously had to be ‘normal’ between birth and what ever date in time (and age) she became roguish, If she hadn’t been ‘normal’ she wouldn’t have been trainable and wouldn’t have become such a nice riding horse. Ergo: She was a ‘good horse’ for quite a long time. And someone thought enough of her to keep her around as a solid riding horse.
That being said, it seems to me that this ‘attacking’ behavior was learned behavior (that reared its ugly head at some point AFTER she was already trained) and came about from some sort of ‘episode’ – caused by an abusive person.
And…this episode, or series of episodes were bad enough to blow this mare’s mind and completely make her distrustful of certain people. MAYBE
Sometimes seriously abused horses who have been abused by MEN, go through their lives hating men. Horses that were abused by women, go through their lives hating women. Sometimes it’s tall people, or short people, or people who wear a certain kind of hat or have a certain kind of voice.
Point being: Certain horses don’t forget the APPEARANCE of that which abused them (say a lead pipe or a whip) and…they don’t forget the physical characteristics of the human being who beat them with the whip or the lead pipe.
Certainly, this type of acute, circumstantial and episodic ‘memory’ (the kind that triggers seemingly bizarre and random violence in a horse, even an abused one) – is RARE. But not unheard of.
Since I don’t know the complete history of this mare’s life, it’s impossible for me to say I’m right about this. But I just keep thinking that this mare is set off by pure fear (of something)–and goes into attack/self preservation mode because of this something.
The fact that she waits and then charges, means to me that she is THINKING before she acts…and then does act because her memory is just too strong to make her change her mind. Bingo–learned behavior she’s taught herself.
I know this offers the OP no advice…
I just feel so bad for this mare (and the OP !! ) that I wish I had the resources to take the mare, in order to TRY to figure out her fears and mental pain.
I’ve seen many batty horses in my time-- all the product of human abuse, or in the case of some racehorses they become savage because they are in pain. Seen that first hand too. Unfortunately.
But–the mare’s problems could have come from something not related to people at all. Perhaps she was never socialized with other horses, and thus never learned how to behave in a herd situation, in the company of alphas and subordinates-- and just doesn’t know her place. Maybe she needs to be turned out with a herd for awhile…so she can learn ‘who’ she is and where she fits, taught by horses in horse language.
Just my thoughts…