Acorns are poisonous to humans (if eaten it can cause damage to your kidneys).
Oak and acorns contain tannins which when ingested produce toxins which can be poisonous to horses. The risk is highest in the UK at the end of long dry summers, even so, most horses are sensible enough not to eat large numbers. It seems that certain individual horses are more susceptible to acorn poisoning than others, some horses can tolerate small numbers of acorns but the toxicity of the acorns can vary from year to year. Acorns and oak contain tannic acid and other tannins that are toxic when consumed in sufficient quantities. These poisons can cause damage to the gut leading to problems such as diarrhea and colic signs, they can also cause damage to the liver and kidneys. There is no specific antidote and the prognosis is poor if kidney damage develops. In rare cases, acorn poisoning can be fatal.