I have three sources to read and watch respectively.
I would like to offer this reflection:
(1) The concept of the behavior of the mare being defined as 'alpha mare' might not be accurate. Studies and anecdotal observations have shown that this kind of behavior is more related to - as with humans - a state of 'psychological pain', unresolved 'trauma', meaning, she's simply not working well with the other horses, it's not that she's the leader. Horses follow confident and fair horses, not one's acting aggressively out of fear.
Read more here:
STUDY: BUSTING THE "LEAD MARE' MYTH https://equusmagazine.com/horse-worl...are-myth-25407
ANECDOTAL OBSERVATIONS & REFLECTIONS: https://horsesenseandcents.com/alpha...u-define-them/
(2) I had a big eye-opener when I found James French and this 'trust-technique'. You don't have to accept everything of his theoretical framework, but I think all of us have exeperienced the shift that happens when we patiently earn the trust of a horse, and two minds connect as one. I think that's what subconsciously brought me to become bosses with horses, once as a 7-year-old girl.
Just watch this video, and yes, you do have to watch it to the end, and then, your curiosity might have been triggered enough to look deeper. Perhaps this mare simply needs some 'healing' to quiet her amygdala reacting out of fear, to be able to connect and become part of the herd in a better way? Sort of like 'collaboration issues' for a human? As Patrick Lencioni and many many others have shown, all collaboration starts with trust, and it seems like this horse is just not there.
(Of course, if someone challenged her, she'd signal to back-off, from a stance of confidence, this just seems over the top, not from a confident alpha mare. I think we're misreading the behavior.)