My best guess is this horse is graveling or an abscess is breaking at the coronary band. This typically happens when an infection enters the white line area of the hoof either through a nail hole or a defect in the hoof wall which allow soil, sand or debris to enter into the softer white line area. This is extremely painful. The infection will work its way up and often break out at the coronary band although it can also bust out at the heel bulb area.
I suggested that you soak the hoof in warm Epsom Salt water for about 20-30 minutes once a day for a few days. After soaking apply Ichthammol to the area on the coronary band that is raised and angry looking—apply Ichthammol at least 2 times a day if possible. Do not apply Ichthammol to the entire coronary band – we want to encourage this abscess to erupt in one spot. The warm water and Epsom Salts will make the area softer and the Ichthammol is a drawing salve that will bring the abscess to the surface and hopefully break open sooner.
Sometimes these horses have a low grade fever from the infection and some even display swelling in the leg. It may be necessary to call a vet and antibiotics may need to be prescribed. I try not to use the antibiotics, because I like the infection to drain. Often when antibiotics are involved, the infection is not completely killed and can just return weeks later. But sometimes antibiotics are just necessary.
Once the infection erupts, then it is important to continue to soak and apply Ichthammol or at the very least just apply Ichthammol because we want to draw all of the infection and not allow the wound to heal before all of the infection is drawn. If the entire infection is not eliminated, you will be right back in the same situation in a few weeks. Once this breaks open, the horse is usually in immediate relief.