All parts of the Oak tree are toxic to horses, the toxic principle is gallotannin. Poisoning usually occurs when horses graze on oak due to lack of adequate forage but some horses do develop a taste for acorns and will seek them out.
Oak poisoning causes gastroenteritis and kidney damage in horses. Symptoms may appear several days after ingestion and include: Abdominal pain, constipation followed by diarrhoea (sometimes bloody), depression, frequent urination, discoloured urine, jaundice, colic, sweating, unsteadiness, fits and coma. Image: Wikimedia Commons
Urgent veterinary assistance is required.
