It is generally not recommended to blanket a wet horse because it can lead to several problems. Here are some reasons why:
Risk of chills and hypothermia: If a wet horse is covered with a blanket, the water trapped between the horse's skin and the blanket will not evaporate, causing the horse's body temperature to drop rapidly. This can lead to chills and hypothermia, which can be dangerous for the horse.
Skin irritation: Covering a wet horse with a blanket can trap moisture and cause skin irritation or even fungal infections.
Increased sweating: If a horse is already wet from sweat or rain, covering them with a blanket can cause them to sweat even more, leading to dehydration and other health problems.
If your horse is wet, it's best to dry them off with a towel and allow them to air dry completely before putting on a blanket. Additionally, if your horse gets wet often, it's recommended to invest in a waterproof or water-resistant blanket that will help keep them dry during wet weather conditions.
Check-out Performance Horse Blankets full Guide to Blanketing Horses.
Checkout Weatherbeeta's Rain Sheet with No Fill for a great Waterproof horse sheet.