Several plants can cause hives or skin irritation in horses:
Stinging Nettle: Tiny hairs cause a burning, itchy rash.
Poison Ivy: Contains urushiol, which can cause allergic reactions.
Poison Oak: Similar to poison ivy and also contains urushiol.
Sumac: Some species can cause allergic reactions in sensitive horses.
Giant Hogweed: Sap can cause severe burns and blisters on contact with skin.