If your horse is exhibiting lameness and one hoof is warmer to the touch than the others, it could be due to a foot abscess, which occurs when foreign matter and bacteria get into the hoof and cause infection. This infection can enter the living tissues of the foot, so it’s vital that you call your vet straight away for advice.
If the abscess isn’t likely to erupt by itself, your vet will probably make a small hole to establish drainage, allowing fluid and pus to seep away from the abscess. Once that has been done, you can poultice the foot yourself as part of the treatment process.