Can acorns kill horses?
Also asked, what can kill a horse quickly?
The most common acute toxins that kill horses in a few hours to 36 hours include: Botulism - often associated with haylage feeding. Yew toxicity - associated with horses consuming clippings from this common ornamental shrub. Poison-hemlock - found in swampy areas.
Besides, can eating an acorn kill you?
It looks like eating raw acorns won't kill you but its not likely to be pleasant and can harm you. (Theoretically you could ruin your kidneys by eating unprocessed acorns, but the tannins make them so nasty that you'd be hard pressed to eat enough to do yourself harm.)
Oak trees–more specifically, their acorns, buds, leaves, or blossoms–are toxic to all livestock, including horses. Horses with oak poisoning can develop colic and bloody diarrhea. They also develop renal disease as the toxic metabolites of the tannins damage the horse's kidneys.