My friend has Morgans. Among them is an 11 y.o. brood mare (ground broke but not saddle broke) - She's been at their farm for 3 years and has been kept in a large fenced area (I'd say at least two-acres) with another mare, a carriage/saddle horse.

She had a foal last year, and was bred back this year, though not sure if she settled, only bred a couple of weeks ago.

To get to the point. This brood mare viciously attacked the saddle horse yesterday. No one saw the attack.  It appears the brood mare must have somehow cornered the more passive mare. Though it's hard to picture that because their paddock is one big square, no strange little places to get cornered.  The damage is bad and mostly to the face, vet patched her up the best he could, but she still has a breathing tube, and of course there's the danger of infection.  Prognosis is questionable.

Anyway, we are all wracking our brains to figure out why this sudden aggressive behavior. She's never shown aggression like this before. She did move to the "boss" a couple of years ago when the oldest mare died. But has never attacked like this before.

She has shown some odd behavior recently like running through double railed fence, for no apparent reason other than she felt like it.

What could possibly make a horse turn suddenly violent?

Ruling out rabies, all are up-to-date. And the mare that attacked is not showing aggression toward humans or any other unusual behavior.

 I just saw some pictures. It looks the broodmare tried to kill this horse. Bite marks to the jugular, tried to tear her ear off, and a hoof mark right under the "victim's" right eye. There's a chunk of flesh that out of the face, complete with teeth marks. I have never seen anything like this before.

An investigation of the pasture shows several areas where scuffles occurred, so the brood mare pursued the saddle horse. By the looks of the fence and the injuries to the saddle horse's legs, it appears she tried to escape by going through the fence but was unable to.

Noone will ever know what happened. 

My policy is to not have dangerous horses on my property.  Now if they spook and run into a fence that's another story but this violent attack would qualify for my "Road Scholarship" program and I'd send her hooves packing.  No animal is worth the safety of a human or their pasture-mates.

To be honest, that's what the owner's dilemma is now. She did have the mare up for sale prior to this. Now, of course, she wants her gone, but could never sell her in good conscience.

Would it be better to put a horse like this down? Or to a place where she'd be the only horse? But, as a seller, you can't always control where the horse will ultimately end up. You still have a responsibility to any potential owner AND a responsibility to the horse.

The mare that was attacked was my friend's personal carriage horse and the horse her daughter was going to show under saddle this year. Those plans are gone. They of course would be devastated with such an attack on any of their horses, but of course, it had to be one that had been in the family the longest.

 If it turns out to be a hormonal thing there's the potential for spaying, or hormone therapy. She's not broke at all though and at 11 years, is it too late? Not worth it? Can she pass this behavior on to her female offspring?

When she arrived she was in foal and I believe she had a filly, which sold. She was bred right away to my friend's stud. She had a foal last year, and was a puppy dog the whole time he was there. The colt was gorgeous, and kind-natured and sold very quickly. They then decided to give her a year off and did not breed her. Over the winter, her behavior slowly changed in that she was just kind of "an air head" as my friend called it but then escalated into this attack.

1.  Fast and done method:  Any auction can take her just put in the notes that this horse is suitable for broodmare only and seems to be an alpha mare.  (To buyers this means she isn't broke, or she's been injured and she's aggressive.)  You've been completely honest and not misrepresented the animal.

2.  Slow and difficult method:  If you really are looking to place her in a good home she needs to be part of a really huge broodmare band and get her rear stomped by other mares daily until she learns her place.  Ask buyers if they have 10+ mares they run together and if they have a super duper mean top mare.  She sounds like she could also run in a program that was pasture bred, since she was lovely with the Stud around.

3.  Give her away method:  If you still feel this isn't exactly what will make the owner happy then donate her to a horse rescue.

4.  Put her down method:  Last resort for me would be to put her down, I've done it and it kills part of your heart even when it's the best/only option.

 What about a possible HORMONE IMBALANCE??? have blood drawn to check her hormone levels... maybe she needs some Regumate or something. Meanwhile, segregate her from the other horses. good luck.

PS: I wouldn't think of selling her like  this...

thanks for all the comments. I will pass them on to my friend.

She reports tonight that her mare is improving in that she's moving around, ears alert, even the one they thought they'd lose. Though her nose is more swollen and both eyes are swollen shut, she's encouraged in that she's more alert.

I don't think she's made any decisions regarding the attacking broodmare, other than to keep her segregated, of course.

This was the first thing I thought of too.  I had a gelding - from the time he was conceived, to 29 yrs of age - that started doing weird things.  He didn't get aggressive  (he was always "ornery", but not agressive) - rain in to & flipped over a woven wire fence, would panic if he thought the gate was closing & he thought he should be on the other side & try to run thru/over it, etc.  I thought he was getting senile.  Then his balance went haywire - started w/a very slight difference in how he placed his right hind foot (I knew this horse better than myself so noticed that right off) & progressed in only a few days to him "leaning" - could not walk in a straight line, would have to lean against the inside of the barn to rest, etc.  Vet was positive it was a tumor.  It was time to say goodbye to him, for his own safety & others.    So I would wonder about this mare & tumor also since you mentioned her going thru the rail fence for no obvious reason.