What could kill a horse within a few hours, no symptoms?

Most likely aneurism or heart attack.

I know of 2 horses that basically dropped dead from an aneurism.

One was a fit, 8 year old Paint stallion. Trainer loped him down the center of the pen, stopped, and the horse dropped.

The other was a 15 year old Appaloosa gelding owned by a friend. She'd ridden him around 'the block' about 2 miles, heading home, he reared and dropped dead, pinning her to the road. Luckily, someone saw it happen and was able to get him off her pretty quickly.

About the only way you will find out is through a necropsy, she could have had a heart attacks, massive stroke or aneurism. The placenta could have pulled away and become toxic to the mare, or possibly a porlapsed uterus that hadn't come out yet. The horse could have spooked in the stall swung her head around and clocked herself dead. I've seen it done before.

I'm sorry for your loss but the only say you will know for sure is the necropsy.

Sorry for your loss.

I saw a horse go from "healthy" to "dead" in just a few minutes, once. This was a valuable horse, and insured, so there was a necropsy. They found an enlarged heart, and concluded "heart failure". (This was a high-level show jumper, who had not shown any symptoms before.)

There's really no way to predict that sort of thing.

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Im sorry to hear that :(. The horse could of got a fright of some kind and died of shock. Heart attack. Anything.

Blister bug posion. Happned to a pony of mine. The only clue was blotches of yellow on his gums and lips.

Heart attack

Internal bleading, if the foal kicked a hole in her.

Lung abcess.

stomach or intestinal abcess burst.

aneurism, CVA, colic, heart attack, gunshot, 22 can barely be seen and no exit wound.

Snake bite?

a necropsy will give you the answer but is extremely expensive.